Senior Democrats accused Republicans in the US Congress of becoming “gelatinous blobs” who won’t stand up to Donald Trump and “rubber stamps” for his war on Iran.

Elizabeth Warren, a US senator for Massachusetts, and Chris Van Hollen, a US senator for Maryland, argued that the Republican-controlled Congress had surrendered its constitutional authority to the president.

In interviews with the Guardian, the senators urged their Republican colleagues to stand with Democrats and assert Congress’s authority as a separate branch of the federal government – with power over declaring war.

“The Republicans have all had their spines surgically removed,” Warren said in Michigan, while stumping for the Senate candidate Abdul El-Sayed. “I mean, they’re just gelatinous blobs that kind of make it through the United States Senate.”

The constitution sets up three separate branches of government to hold one another accountable, Warren noted – and the assumption was that each of the three branches would “jealously guard its own powers”.

“But with Donald Trump in the White House, and a bunch of weak-kneed Republicans in the House [of Representatives] and the Senate, all of a sudden, we’ve lost the whole game,” she said.

The US-Israel war on Iran, launched in February, has cost the US tens of billions of dollars. Hundreds of US service members have been wounded and 18 have been killed since the start of the operation. A tentative peace deal fractured in recent weeks.

The latest congressional bid to rein in Trump’s attacks on the Middle East failed on Thursday, when a war powers resolution failed by 49 votes to 50 in the Senate.

The controversial conflict rumbles on, as Trump faces a crucial political test with November’s US midterm elections. Democrats could win back power in the House, which is currently controlled by Republicans, and are also vying to retake control of the Senate.

If there is a Democratic majority, Warren suggested that, coupled with the groundswell of opposition to the war among voters, could enable Congress to serve as a proper check on Trump.

If that happens, “I think we are able to rein in Donald Trump and [defense secretary] Pete Hegseth, even if they have gallons of testosterone”, said Warren.

Earlier this month, the House passed a war powers resolution for the second time that directed Trump to withdraw the US military from Iran – this time on a 214-208 vote, with four Republicans joining all Democrats in voting in favor. But in the Senate, the resolution failed 47-49.

In June, a similar war powers resolution passed the Senate. The resolution directs the USmilitary be removed from Iran without explicit approval from Congress. Van Hollen, who introduced the war powers resolution, said the situation in Iran was a “senseless war” that “should never have started”.

“Ultimately, Congress will have to turn off the funding spigot for this war,” Van Hollen, also in Michigan to support El-Sayed, said. “And I will not vote for a dime for this illegal war,” adding that he wouldn’t support Hegseth’s request for more funding, delivered to the appropriations committee earlier this month.

“At some point, these Republican senators are going to have to explain to their constituents why they’ve contracted out their constitutional responsibilities to Donald Trump, because that’s what they’re doing,” saidVan Hollen. “So long as you have a majority of senators who are turning over their constitutional responsibilities to Donald Trump, then they’re just rubber stamps for Donald Trump’s illegal war.”

Van Hollen said Democrats should continue to press for votes on the war powers resolution, to hold Republican senators accountable.

The war is unpopular in the US, with 29% of Americans approving of Trump’s handling of the situation with Iran, and 69% disapproving, according to a Washington Post-Ipsos poll earlier this month.

“I would rather see Republicans assert their constitutional duties and help us end the war now,” added Van Hollen. “But, you know, maybe they’ll get the message from their constituents.”