U.S. Senate Democrats on Tuesday lashed out at President Donald Trump's "unconstitutional" war on Iran, urging Republicans to "grow a spine" and vote with Democrats to rein in the president's war powers ahead of an expected war powers vote in the Senate.

"It's been four days since the Trump administration launched the U.S. into a war most Americans oppose, and his administration can't explain the reasons we did it. I left the intelligence briefing more concerned than reassured," Senate Minority Leader Chuck Schumer told reporters at a press briefing on the Capitol Hill.

"The rationales change by the hour, regime change, nuclear weapons, missiles, defense, preemptive, which is it? When the justification keeps shifting, a strategy is missing. There is no strategy," said Schumer.

Senator Patty Murray, vice chair of the Senate Appropriations Committee, who also attended the press briefing, said that "Donald Trump started an unconstitutional war that no one asked for."

"Congress should end this war now. Republicans should grow a spine and vote with Democrats in support of the War Powers resolution," Murray said.

The War Powers Act is a 1973 U.S. federal law designed to limit the president's ability to commit U.S. forces to armed conflicts without Congressional approval.

The law restricts military action without congressional approval to a narrow circumstance: "a national emergency created by attack upon the United States, its territories or possessions, or its armed forces."

While the law gives Congress a fast track to curb the president's war powers through a joint resolution, it remains unclear whether such a resolution concerning Iran could garner enough support to clear both chambers, considering that most Republicans support the U.S. and Israeli strikes on Iran.

The Senate is expected to vote on the war powers resolution as soon as Wednesday.

Senate Democrats also argued that the Trump administration's priorities are misplaced, spending large amounts of money on foreign wars while failing to address urgent domestic issues such as soaring health care and grocery prices.

"Haven't we learned anything from 25 years in the wars in the Middle East? More than 14,000 U.S. troops and contractors killed, more than 65,000 troops and contractors injured, hundreds of thousands of civilian deaths, 8.2 trillion dollars that could have been spent here at home, spent in wars in the Middle East," said Veteran Democratic Senator Tim Kaine, who also participated in the press briefing.

"When you use your money to wage war, you take money away from other priorities," said Kaine. "When you wage a war that raises energy prices that are already too high, you're hitting people who are dealing with health care costs up, housing up, food up, groceries up, energy costs up."

Meanwhile, Senate Republican leaders continued to defend the president's decision.

"I think the president was perfectly within his rights to take the steps that he took. I think it was a necessary step in order to protect American lives," Senate Majority Leader John Thune told reporters at a separate press conference on Capitol Hill.