Members of the Senate and House of Representatives argued that US intelligence had not shown an imminent threat from the Middle Eastern country that justified Trump's unilateral action.

"President Trump's actions in bombing Iran puts the US on the brink of a wider war in the Middle East, all without constitutionally required Congressional approval," Senate Democratic Whip Dick Durbin said in a statement.

Democrats were divided between those demanding a vote on a war powers resolution to constrain Trump's authority to launch further action and a smaller group, who maintained that the strikes were grounds for the Republican leader's impeachment.

They included Illinois moderate Sean Casten and New York leftist Alexandria Ocasio-Cortez, who accused the president of having "impulsively risked launching a war that may ensnare us for generations."

Chuck Schumer and Hakeem Jeffries, the Democratic leaders in the Senate and House respectively, said Trump had "dramatically increased" America's risk of becoming embroiled in a new Middle Eastern conflagration.

"No president should be allowed to unilaterally march this nation into something as consequential as war with erratic threats and no strategy," Schumer said.

The Democrats have foreign policy hawks in their ranks and many were quick to point to the threat that a nuclear-armed Iran would pose --- while still upbraiding Trump for acting without consulting lawmakers.

"The Constitution makes clear that the power to authorize war lies with Congress... The American people deserve more than vague rhetoric and unilateral decisions that could set off a wider war," said Mark Warner, vice chairman of the Senate intelligence committee.

The loudest Democratic voice in support of the strikes was staunchly pro-Israel Senator John Fetterman, who singled out Trump for praise -- something even party colleagues who support the strikes have avoided.

"As I've long maintained, this was the correct move by (Trump)," the Pennsylvania centrist posted on X. "Iran is the world's leading sponsor of terrorism and cannot have nuclear capabilities."

Republicans have been lining up since the strikes to praise Trump and endorse his decision to hit three Iranian nuclear facilities -- with little dissent among the ranks.

But Kentucky conservative Thomas Massie accused Trump of escalating the conflict between Israel and Iran.

"When two countries are bombing each other daily in a hot war, and a third country joins the bombing, that's an act of war," said Massie, who introduced a bipartisan resolution earlier this month to require any military action to be approved by lawmakers.

"I'm amazed at the mental gymnastics being undertaken by neocons in DC (and their social media bots) to say we aren't at war... so they can make war."