President Donald Trump has recently floated a series of highly unconventional foreign policy ideas, from annexing Greenland and retaking the Panama Canal, to imposing hefty tariffs on imports from every country in the world, to using the military to deport more than 10 million undocumented immigrants. He also has a track record of praising authoritarian leaders, seeking to gut the State Department, and questioning whether he would uphold the U.S. security commitment to NATO.
To what extent can the U.S. Congress serve as a check on Trump’s most disruptive foreign policy instincts?
With the Republican Party in control of the House of Representatives and Senate, the odds of wide-ranging congressional pushback against Trump appear slim. Nevertheless, there will likely be important exceptions as some Republican lawmakers join with Democratic counterparts to try to maintain guardrails around U.S. foreign policy. These guardrails could include the preservation of security alliances, rejection of territorial conquest, and protection of the budget for foreign aid. In doing so, they could soften the edges of Trump’s America First agenda and limit the chaos that it threatens to create.
During Trump’s first term as president, influential congressional Republicans cooperated with Democrats to protect some of these guardrails. For example, when Trump proposed slashing U.S. spending on diplomacy and foreign assistance by one-third, Republican Senator Lindsey Graham and Democratic Senator Patrick Leahy, along with their counterparts in the House, used their committee leadership positions to maintain the international affairs budget at a roughly constant level. Similarly, when Trump questioned the value of NATO, then-Senate Majority Leader Mitch McConnell and then-Speaker of the House Paul Ryan backed bipartisan resolutions reiterating U.S. support for the alliance. Congress further strengthened this NATO guardrail in 2023 with a bipartisan measure introduced by Democratic Senator Tim Kaine and Republican Senator Marco Rubio that prohibits the president from withdrawing from NATO without congressional assent. Rubio is, of course, now Trump’s secretary of state.
Democrats and Republicans have also worked together to protect the U.S. security commitment to South Korea. During his first term, Trump’s attempt at dealmaking with North Korean leader Kim Jong Un and pressure on South Korea to spend more on its own defense generated concerns that he might abandon Seoul. Members of the armed services committees responded by adding to annual defense legislation a provision that constrained the president’s authority to reduce U.S. troop levels on the Korean Peninsula.
More recently, Congress included in fiscal year 2025 defense legislation a provision calling on the United States to maintain 28,500 troops in South Korea and affirming America’s extended deterrence commitment to the ally. Republicans and Democrats have been able to reach agreement on these measures and approve them without controversy because there remains strong support for alliances across the aisle. A 2024 survey of foreign policy opinion leaders by the Chicago Council on Global Affairs found that 71 percent of Republicans and 83 percent of Democrats think alliances benefit both the United States and our allies.
Compared to Trump’s first term, a greater share of Republicans in Congress now identify with Trump’s Make America Great Again (MAGA) movement and follow his lead across the board. After Trump suggested acquiring Greenland and the Panama Canal, the House Foreign Affairs Committee, now led by MAGA-aligned Republican Brian Mast, posted on X, “Our country was built by warriors and explorers. We tamed the West, won two World Wars, and were the first to plant our flag on the moon. President Trump has the biggest dreams for America and it’s un-American to be afraid of big dreams.” Trump also successfully lobbied Speaker of the House Mike Johnson to change the leadership of the House Intelligence Committee at the start of this Congress, replacing Mike Turner, a hawk who has challenged Trump on Ukraine policy and some intelligence issues, with Mike Crawford, a consistently staunch defender of Trump who has opposed further U.S. aid to Ukraine.
While most House Republicans will act as cheerleaders for Trump on foreign policy, the story will be more mixed in the Senate. Traditional Reaganites, who value U.S. military primacy, moral leadership, and the robust use of all instruments of national power, from covert action to foreign aid, remain in key positions of leadership in the upper chamber. John Thune, the new Republican Senate majority leader, has been a consistent supporter of alliances as well as funding for diplomacy and foreign assistance. Senators Roger Wicker, McConnell, and Graham, who also fall squarely in the Reaganite tradition, are serving, respectively, as the chairmen of the Armed Services Committee and of the appropriations subcommittees for military and civilian foreign policy spending. Strikingly, after agreeing to step down as the Senate Republican leader last year, McConnell noted that he planned to devote his remaining two years in the Senate to securing “U.S. leadership and primacy” and protecting the Senate’s institutional independence. In a recent Foreign Affairs article, he also argued that “the United States cannot neglect the role of foreign aid” in advancing U.S. security—a striking contrast to an executive order issued by Trump on the second day of his new term that placed a ninety-day freeze on nearly all foreign aid spending.
To be sure, on many foreign policy issues, there will not be much daylight between Trump and Republican senators. Republicans are united in favoring the placement of heavy economic pressure on China, Iran, Cuba, and Venezuela; strong support for Israel; hardening the border with Mexico; and the removal of many policies designed to address climate change. Moreover, on some of these issues, such as the application of pressure against China and the continuation of military aid for Israel, many Democrats are on the same page as their Republican colleagues.
But on some other issues, intraparty fractures within the Republican Party may be pronounced. While most Republicans in Congress are willing to give Trump considerable leeway to negotiate an agreement to end the Russia-Ukraine war, pro-Ukraine Republicans, such as Thune, McConnell, Wicker, and House Armed Services Committee Chairman Mike Rogers, may push back against Trump if he moves to reach a deal with Russian President Vladimir Putin that they think does not adequately protect Ukraine. Consider that, as recently as last April, thirty-one Senate Republicans and 101 House Republicans joined forty-eight Senate Democrats and 210 House Democrats in voting to approve $61 billion in new military aid for Ukraine—even though Trump had opposed further U.S. assistance to the country.
Of course, most Republicans in Congress are reluctant to challenge Trump in a public way, fearful of how he might retaliate against them politically by backing Republican primary challengers when they stand for reelection. But some senior Republicans—particularly ones who maintain a strong constituency separate from the MAGA movement, possess an independent streak, or will not be running for reelection—may be willing to go further than the rest of the flock.
For example, if Trump seems to be abandoning Ukraine, don’t be surprised to see McConnell denouncing the president’s policy on the Senate floor. McConnell and Wicker could also use their powerful committee positions to try to ensure continued U.S. support for Ukraine by appropriating further military aid for the country or legislating a prohibition on U.S. recognition of Russian control of occupied Ukrainian territory unless Ukraine agrees to it. Even if such legislative efforts are unsuccessful in the face of Trump’s opposition, just the prospect of having his policy publicly challenged by high-profile Republicans might cause him to negotiate with Putin differently. Indeed, Trump’s recent tough-sounding rhetoric toward Putin may be motivated in part by a desire to insulate himself from the potential criticism that he accommodated the Russian president too much.
On a related transatlantic issue, Trump may be deterred from pulling out of NATO by the 2023 law that prohibits U.S withdrawal from the alliance without congressional approval of the step. Congress could further strengthen the U.S. commitment to European allies by prohibiting the president from reducing the number of U.S. troops deployed to NATO countries below a certain number without Congress having an opportunity to weigh in on the proposed change. To be sure, Trump could still greatly weaken U.S.-NATO relations in other ways—for instance, by refusing to send a representative to NATO meetings—but legislative constraints could limit the long-term damage to the alliance.
Trump may also face pushback from both parties in Congress if he pursues the more aggressive pieces of his protectionist trade agenda. He has proposed imposing a tariff of 10 to 20 percent on all imported goods, as well as large additional tariffs on imports from China, Mexico, Canada, and countries that undermine the U.S. dollar’s status as a global reserve currency. In response to some of these proposals, Republican Senator Mike Rounds of South Dakota, which relies heavily on agricultural exports, commented in November, “Tariffs have a place, but there is also the other side, which is: what about the repercussions of [countries] putting tariffs in a retaliatory method on some of our items as well?” If Trump goes beyond the limited use of tariffs to impose levies in a manner that will create major economic losses to important U.S. constituencies, Republicans and Democrats representing those constituencies could join together to advance legislation that would give Congress a greater say in tariff decisions—for instance, by giving Congress an opportunity to endorse or reject certain proposed tariffs before they go into effect. Whether such a legislative effort succeeds may depend on whether Republicans representing pro-trade agricultural constituencies are able to bring other Republicans on board.
Similarly, while most Republicans back Trump’s plans to seal the border and deport undocumented immigrants, some GOP lawmakers will oppose any moves to deploy the military to round people up. In November, Republican Senator Rand Paul stated, “I will not support and will not vote to use the military in our cities. I think it’s a terrible image.” Republicans and Democrats concerned about the potential use of the military for deporting immigrants could advance legislation clarifying that the 1807 Insurrection Act does not authorize the president to use the military for the purpose of enforcing immigration laws.
Republicans who oppose military interventionism could also join with Democrats to constrain the president from using force against Panama or Greenland. In 2020, when members of Congress grew concerned that Trump might take military action against Iran, the House and Senate passed a resolution, with backing from eight Senate Republicans and six House Republicans as well as nearly all Democrats, that prohibited the use of force against Iran unless Congress authorized it. Although Trump vetoed the resolution, it signaled that Congress would not be behind the president if he chose to pursue war, and this may have dissuaded him from carrying out military action. In a similar way, members of Congress in both parties who oppose interventionism or consider it important to protect Congress’s constitutional war powers authority, such as Kaine and Republican Senator Todd Young, could advance legislation that prohibits the use of force against Panama or Greenland without congressional approval. In one sign of congressional opposition to annexing Greenland, Republican Senator Lisa Murkowski recently issued a joint statement with a Danish member of Parliament stating that Greenland is “an ally, not an asset.”
None of this is to say that Congress is going to take over the reins of U.S. foreign policy. To the contrary, the president will remain the dominant actor in foreign affairs, continuing a decadeslong trend of growing executive power, as it will remain difficult on many issues for Congress to overcome its internal divisions. But Congress is still capable of maintaining some guardrails around foreign policy. Most importantly, some Republican lawmakers may still be willing to join with Democrats in efforts to preserve core elements of internationalism, and those efforts could temper Trump’s foreign policy in important ways.
