THE SENATE HAS approved a bi-partisan measure aimed at limiting President Donald Trump’s authority to launch military operations against Iran.

Eight Republicans joined Democrats in a post-impeachment bid to constrain the White House.

The rebuke was the Senate’s first major vote since acquitting Trump on impeachment charges last week.

Trump is expected to veto the war powers resolution if it reaches his desk, warning that if his “hands were tied, Iran would have a field day”.

The measure, authored by Democrat Tim Kaine, says Trump must win approval from Congress before engaging in further military action against Iran.

Kaine and other supporters said the resolution, which passed 55-45, was not about Mr Trump or even the presidency, but instead was an important reassertion of congressional power to declare war.

While Trump and other presidents “must always have the ability to defend the United States from imminent attack, the executive power to initiate war stops there,” Mr Kaine said. “An offensive war requires a congressional debate and vote.”

The Senate vote continues a pattern in which Republican senators have shown a willingness to challenge Trump on foreign policy, a sharp departure from their strong support during impeachment and on domestic matters.

Congress moved to impose restrictions on US involvement with the Saudi-led war in Yemen last year after US-based journalist Jamal Khashoggi was killed in a gruesome murder at Saudi Arabia’s consulate in Turkey.

The bi-partisan vote was a rare exertion of authority from Congress, the first since passage of the War Powers Act of 1973 and Trump promptly vetoed it.

The Democratic-controlled House passed a separate, non-binding war powers resolution on Iran last month. The House could take up the Senate resolution later this month, House leaders said.

Two-thirds votes in the House and Senate would be needed to override an expected Trump veto of the war powers resolution.

Answering a claim by Trump that the measure would send a signal of weakness to Iran and other potential adversaries, Kaine said the opposite was true.

“When we stand up for the rule of law … and say ‘This decision is fundamental, and we have rules that we are going to follow so we can make a good decision,’ that’s a message of strength,” Kaine said.

“If we’re to order our young men and women … to risk their lives in war, it should be on the basis of careful deliberation by the people’s elected legislature and not on the say-so of any one person.”

Republican Mike Lee agreed and said he supports Trump’s foreign policy, including toward Iran, but said Congress cannot escape its constitutional responsibility to act on matters of war and peace.

As the Senate debate made clear, “there is abundant support for the United States taking tough positions with regard to Iran”, Lee said. ”And as part of that we want to make sure that any military action that needs to be authorised is in fact properly authorised by Congress. That doesn’t show weakness. That shows strength.”

Trump disputed that, arguing on Twitter that a vote against Kaine’s proposal was important to national security and pointed to the drone strike that killed Iran’s top general, Qassem Soleimani.

“We are doing very well with Iran and this is not the time to show weakness. Americans overwhelmingly support our attack on terrorist Soleimani,” Trump said. “If my hands were tied, Iran would have a field day. Sends a very bad signal. The Democrats are only doing this as an attempt to embarrass the Republican Party. Don’t let it happen!”