"It's important for the president, before we even cross over into the new Congress, to end this war in Iran," House Democratic leader Hakeem Jeffries told MS Now's Morning Joe programme.
Jeffries blamed what he called Trump's "reckless and costly war of choice in Iran" for driving up fuel and fertiliser costs, contributing to higher prices for groceries and other goods.
The Democrats' victories in Tuesday's elections gave them control of both the House of Representatives and Senate, reversing Republican majorities and potentially allowing them to constrain President Donald Trump's agenda during the final two years of his presidency.
Democrats flipped at least seven Republican-held Senate seats, winning in Texas, Kansas, Iowa, Ohio, Maine, North Carolina and South Carolina. They also captured more than two dozen Republican-held House districts, including areas Trump had won comfortably in the 2024 presidential election.
Among the most notable results, James Talarico became the first Democrat elected to the Senate from Texas since 1988, defeating Republican Ken Paxton. Democrats also retained competitive Senate seats in Georgia and Michigan.
The scale of the victories reflected voter dissatisfaction over high living costs, the war with Iran and concerns about government corruption, according to Associated Press voter polling.
Democratic leaders demand an end to Iran war
Senate Democratic leader Chuck Schumer said the incoming majority could use congressional control over government spending to push for an end to military operations against Iran.
"This war must end. It must end," Schumer told reporters Wednesday.
He said Congress's "power of the purse" was one way to restrain a president conducting a war without the consent of the American people.
Jeffries, who is set to become House speaker when the new Congress convenes in January, said the party's priority would be reducing the cost of groceries, gasoline, housing and healthcare.
"We ran on affordability. We won on affordability," he said.
Trump and Vance acknowledge war's impact
Vice President JD Vance blamed the Iran war for Republicans' losses, writing on X that "the reason we lost is the war."
Trump acknowledged Wednesday that the conflict had likely contributed to the electoral defeat by pushing up gasoline prices, but defended his decision to launch military action against Tehran.
"I think it's largely true because it drove the gasoline up a little bit short term," Trump said when asked about Vance's assessment.
He insisted the war had been necessary to prevent Iran from acquiring nuclear weapons and said he would make the same decision again.
"I can tell you that as far as that war is concerned, I would have done nothing different," he said.
Trump also argued that Republicans would have won if he had been on the ballot, saying the enthusiasm he attracted at campaign rallies had not transferred to other Republican candidates.
The Democratic takeover of Congress will give the party greater power to investigate the Trump administration, challenge its legislative agenda and potentially restrict military funding.
However, Trump will retain the presidency and veto power, meaning congressional control alone will not give Democrats the authority to end the war.
Tehran hails results, warn against further strikes
Iranian officials portrayed the Democratic victories as a rejection of Trump's military campaign, although Tehran remained wary of renewed US attacks.
"Conscience or the cost of living, Americans said NO TO WAR, and we endorse this message," Parliament Speaker Mohammad Bagher Ghalibaf wrote on X Wednesday.
Ghalibaf, who has led Iran's negotiations with Washington, warned the US government against disregarding voters' message by launching further strikes.
Supreme Leader Mojtaba Khamenei, however, dismissed expectations that a change in the balance of power in Washington would alter US hostility toward Tehran.
A written statement attributed to Khamenei warned against "even the slightest inclination towards submission" to the United States, saying Washington's approach would persist regardless of which party was in power.
"The US sees the end of its hegemony as distant; we see it as near," Khamenei added.
Two senior Iranian sources cited by Reuters said that while Tehran welcomed the Democratic victories, Iranian leaders feared Trump could attempt to compensate for his electoral setback with renewed military action.