In a rare occurrence, a US election may have actually been decided by foreign policy. Donald Trump’s war in Iran and international tariffs are seen as playing a key role in the inflationary pressures that drove so many Americans to vote against his party on Tuesday in what has been described as a “generational wipeout” for the Republicans.

And in the coming months, the president is going to face severe headwinds on those policies as Democrats storm back into control of Congress, threatening investigations of some of his top envoys and Pentagon contractors, big fights over budgets linked to the Iran war, and greater scrutiny of US relationships with allies such as Israel.

While Democrats plot their strategy, the civil war within the Republican party has already begun. JD Vance was early out of the gate, announcing in a postmortem on X that the Republicans’ shellacking was directly linked to Trump’s decision to strike Iran. “The reason we lost is the war,” he said. “The president himself said so last night.” Soon Donald Trump Jr was chiming in: “Every word of this is 💯💯💯.”

Trump Sr partly confirmed the vice-president’s analysis, admitting that the war had pushed gasoline prices up. But he added that he believed the Iran war was worth it, and that “we had to do it, we continue to do it”.

“Long term, it’s the only answer,” he said. “You can’t let Iran have a nuclear weapon.” Seeing little left to lose, he may resume strikes in the upcoming months, seeking to secure his legacy with fewer concerns about voter backlash with the midterms already passed.

Democrats will hold a majority in both houses of Congress, but won’t be able to override a presidential veto to force legislation into law. So the likely battles in the coming months will be focused on finding leverage to constrain the White House – most clearly through Congress’s control of the budget.

“The Democrats taking control of both houses gives them unprecedented leverage – especially using the budget,” said Ryan Crocker, a former career diplomat and diplomatic fellow at the Middle East Institute, a thinktank. “We have seen the [Pentagon] request to dramatically increase funding to support the war with Iran come into substantial criticism from Democrats, and I think that will be the first battleground.”

Democrats will have an early opportunity to pressure the Trump administration during the lame duck session of Congress in December, when funding for the government through a continuing resolution is set to expire. They could also hold hostage the passage of the National Defense Authorization Act, demanding concessions on Iran or limits on Trump’s authority in order to pass the Senate version of the bill that would authorise about $1.15tn in national defense spending for 2027.

One progressive foreign policy thinktank suggested inserting language into the continuing resolution that would prevent Trump from using the money authorised by Congress to fund the war in Iran, as well as in Yemen, Lebanon and Gaza. But shutting down the government could be a “two-edged sword”, said Crocker, and Democrats will have to be careful not to overplay their hand.

“American voters were pretty angry in this midterm election – they were voting against President Trump and his actions and policies as much as they were voting for [Democrats] – and that means that Democrats will have to be careful how they use this power.”

Democrats had little success in 2019 restraining Trump from unpopular foreign policy decisions like an abrupt withdrawal from Syria that abandoned Kurdish allies or to end his support for the Saudi war in Yemen. When Congress managed to pass a bipartisan resolution that invoked the War Powers Act to end US support for Saudi Arabia in the deadly war, Trump simply vetoed it.

Even with control of both houses of Congress, it will remain difficult for Democrats to constrain Trump’s power to dictate foreign policy over tariffs, the war in Iran or aid for Ukraine such as intelligence sharing, deliveries of desperately needed Patriot interceptors or access to the Starlink satellite internet service.

“It is worth watching how and where the new Congress chooses to convert its investigative energies into real policy constraints on the executive branch,” wrote Jennifer Hendrixson White, a visiting scholar in the American Statecraft Program at the Carnegie Endowment for International Peace. “Congressional investigations into the Iran war or defense contracts with Trump-linked firms could pressure or embarrass the president, but it is less clear whether a Democratic-controlled House or Senate would be able to convert those investigations into meaningful actions.”

Still, the elections have upended the status quo for foreign policy in both parties, and Trump will increasingly face pressure from his own allies to end the war in Iran. Vance, in his public statement about the elections, prescribed his cure for what is ailing the Republican party: “1) End the war 2) Focus on cost of living/inflation 3) Put America First 4) Change our relationship to Israel.”

That also marks a dangerous moment for Israel, wrote Amit Segal, a political analyst close to the Netanyahu administration, which “now faces its biggest challenge in Washington since at least the 1975 ‘reassessment’, when Ford and Kissinger froze arms deals to pressure Jerusalem,” he wrote.

Abdul El-Sayed, the Michigan Democrat who will be the first sitting Muslim senator next year, illustrated how far US policy has shifted in this election as he praised Trump’s decision to end military aid to Israel at the end of this year. “I think the president is actually right on this policy,” he said. “I’d also like to see us stop selling weaponry that gets used to do the worst things, including genocide and apartheid, to Israel.”