The historic “blue wave” in the US midterm elections will have consequences extending well beyond domestic battles in Washington. For only the fourth time since 1946 has the incumbent party lost both houses. We have seen such congressional shifts impact foreign policy before. For example, the Clinton administration’s efforts to end North Korea’s nuclear program in part collapsed after Democrats lost control of both houses in the “Republican Revolution” in 1994.

Losing Congressional majorities may not necessarily change President Trump’s underlying foreign policy preferences, but it will certainly make it more difficult to enact policy without congressional opposition.

The implications are particularly important for the ongoing war with Iran, American support for Ukraine, and the future of US relations with Taiwan. Each illustrates how Congress can serve as an institutional constraint on presidential decision-making. The scale of Republican losses and Trump’s own lame-duck status will likely make congressional Republicans cautious in supporting policies that undermine their own future election ambitions.

The Political Costs of War

Despite the claims made by Vice President JD Vance and others, the war with Iran was not the principal reason behind Republican congressional losses, as exit polls typically put concerns about the cost of living and government corruption as higher concerns. However, polls consistently showed Americans questioned the decision to use military force against Iran, and the conflict's economic consequences complicated separating the administration’s foreign policy decisions from voters' everyday concerns. Trump's own response to criticism of the war only exacerbated issues, including dismissing American casualties by comparing the conflict to casualties during the Vietnam War.

This tendency to dismiss the complexity and potential consequences of military engagements has become a recurring feature of Trump's foreign policy. Just prior to the election, Trump claimed at a rally that he could have won the Vietnam War in a week, echoing similar claims he made in 2024 about ending the war in Ukraine within 24 hours. The problem is that military conflicts rarely conform to such expectations, or to quote the boxer Mike Tyson, “Everyone has a plan until they get punched in the face.” The Iran war has demonstrated how quickly military operations can generate broader economic and strategic consequences. 

A Democratic-controlled Congress will have considerably more opportunities to challenge the administration's approach on Iran. Democrats can use committee investigations, hearings, and the War Powers Resolution to scrutinize the justification and continuation of military operations in Iran. Congress may even call for the impeachment of Secretary of War Pete Hegseth. More importantly, since Congress controls federal appropriations, it can limit or deny future funding requests or attempt to limit existing funds to specific military activities. While it is unclear whether Congress will push the administration to end the war, Congressional pressure may encourage Trump to pursue means to save face through a negotiated settlement or drastically reduce the scope of future operations.

Ukraine and the Limits of Presidential Discretion

Trump has repeatedly expressed frustration with the financial and military costs of supporting Ukraine, while occasionally making statements that appear sympathetic to Russian President Vladimir Putin's position. His willingness to threaten reductions in American assistance and to support negotiations that could require substantial Ukrainian territorial concessions raise concerns about Ukraine’s continued resistance to Russian aggression. 

Congressional support, however, remains strong and will likely increase in 2027. The House passed in June the Ukraine Support Act with 18 Republicans joining most Democrats in support. The new Democratic majority should make it easier to promote continued assistance to Ukraine, even if support is unlikely enough to override a presidential veto. Short of this, Congress can provide greater scrutiny and thus political costs for the administration if the latter seeks to reduce assistance, ease sanctions on Russia, or pressure Ukraine to accept settlements not in their own interest. In other words, congressional opposition could make it harder for the administration to abandon Ukraine without facing a prolonged political fight. In doing so, this may allay some concerns from European allies and Ukraine about the depth of American commitments here.

Taiwan and the Durability of Congressional Support

For Taiwan, the midterm elections offer some reason for reassurance, although they do not eliminate the uncertainties surrounding Trump's approach to cross-Strait relations.

The administration has continued to approve substantial arms sales to Taiwan, including a record $11 billion package in December 2025. However, Trump's comments raised questions about the consistency of American policy, both in terms of the willingness to defend Taiwan but whether the administration would use concessions regarding Taiwan as a negotiating tool with China. 

Congress, however, has traditionally demonstrated bipartisan consistency regarding Taiwan and continued defensive arms sales and efforts to strengthen Taiwan's ability to deter Chinese military aggression. Recent congressional initiatives illustrate this continuing commitment. In May 2026, Democratic and Republican senators introduced a bipartisan resolution reaffirming the Taiwan Relations Act. A separate bipartisan group of senators also pressed the administration to move forward with a previously approved $14 billion arms package. These efforts demonstrate that congressional support for Taiwan extends beyond the administration's own policy preferences.

A Democratic-controlled Congress will likely continue to provide an important institutional counterweight to any attempt to reduce support for Taiwan. The return to the Senate of Sherrod Brown, one of the founding co-chairs of the Congressional Taiwan Caucus in 2002, should only strengthen this commitment. For Taiwan, the distinction between congressional support and presidential policy will likely become more important. Despite Trump’s penchant for transactional foreign policy, Congress is likely to remain an important source of support.

While conditions may seem to favor continued congressional support, Taipei should view the new Congress as an opportunity to deepen relations, particularly with lawmakers who may be skeptical of American international commitments or who do not immediately see how relations with Taiwan benefit their own constituents. Traditional engagement efforts with members of the House and Senate Foreign Relations committees remain important, but myopically focusing on these committees overlooks potential opportunities to cultivate relationships with a wider range of lawmakers.

This will require making the case for Taiwan in terms that resonate beyond national security concerns. To some, Taiwan’s pivotal role in semiconductor production and technology supply chains offers opportunities to connect with legislators who concerns fall on domestic economic development. However, Taiwan should also seek to expand relationships with members representing agricultural and industrial constituencies that could benefit from expanded commercial ties. The point is that such efforts help shift the framing of relations away from primarily security concerns to one about tangible shared economic interests. 

Taiwan must also demonstrate how it is prepared to contribute meaningfully to its own defense. Increased defense spending and expanded domestic defense production can help reassure American lawmakers that Taiwan takes the security challenge seriously. However, this requires also addressing Taiwan's partisan divisions. President Lai Ching-te's proposed defense budget has encountered considerable opposition from the Kuomintang (KMT) and Taiwan People's Party (TPP), who together form a majority in the Legislative Yuan. Taiwan cannot afford every defensive option and proposed spending deserves heightened criticism. Nevertheless, framing defense spending as a demonstration of Taiwan's commitment to its own security, and not just a response to American pressure, could help navigate partisan divisions. Such a move potentially strengthens Taiwan's credibility with members of Congress expecting Taiwan to shoulder more of its own defense responsibilities.

Taiwan should also approach congressional engagement as a form of long-term strategic insurance. While the primary goals should be maintaining bipartisan relationships, these relationships become a particular source of resistance if President Trump now pursues a more transactional relationship with Beijing at Taiwan's expense. It could also serve Taiwan well beyond Trump's presidency, especially as several members of Congress will likely seek their party's presidential nomination in 2028 or later. Building broad relationships with lawmakers now would be an investment in the longer-term stability of US-Taiwan relations.

The Power of the Purse

Perhaps the most consequential way the new Congress can constrain Trump is through its control over federal spending. Despite the attention on rhetoric and meetings, foreign policy implementation depends heavily on congressional appropriations. This gives Congress considerable leverage, particularly when an administration is pursuing expensive military operations or seeking to substantially alter existing foreign assistance commitments.

The Iran war provides an especially clear example. Continued operations, replenishment of weapons stockpiles, and additional military deployments all carry substantial financial costs. The administration's requests for supplemental funding will therefore create opportunities for Congress to influence the scope and duration of the conflict. The same principle applies to Ukraine. Congress can support assistance programs even when the administration is reluctant to do so, while also scrutinizing proposed reductions in funding. The budget may ultimately prove to be a more effective instrument of congressional influence than attempts to directly challenge Trump's authority over foreign policy.

The Return of Congressional Checks 

The most important consequence of the 2026 midterm elections may ultimately be the political environment created for the remainder of Trump's presidency.

The scale of Republican losses should concern Republican lawmakers who must consider their own political futures. Members of Congress facing reelection in 2028, or considering their own presidential bids, will have little incentive to defend policies that have become unpopular, particularly if those policies involve expensive military operations or appear to undermine longstanding American security commitments. This does not mean that congressional Republicans will suddenly abandon Trump or that Democrats will agree on a unified foreign policy agenda. But agreement on every aspect of foreign policy is unnecessary for Congress to constrain the president. Different bipartisan coalitions can emerge around specific issues, whether limiting military operations in Iran, maintaining assistance to Ukraine, or preserving support for Taiwan.

Trump will also retain important advantages. He remains commander in chief, controls the executive branch's diplomatic machinery, and can veto legislation. A Democratic Congress will not be able to simply dictate American foreign policy.

Nevertheless, the combination of Democratic control of both chambers, greater oversight authority, and Republican electoral vulnerability will create a substantially different environment for presidential decision-making in 2027.

For America's allies and partners, the elections offer the prospect of a more active Congress willing to defend American commitments and challenge presidential decisions.