WASHINGTON (AP) — A bipartisan measure limiting President Donald Trump's authority to launch military operations against Iran is moving toward approval in the Senate.

The resolution, authored by Democratic Sen. Tim Kaine of Virginia, asserts that Trump must win approval from Congress before engaging in further military action against Iran. Eight Republicans sided with Democrats Wednesday on a procedural motion to force a vote on the issue as soon as Thursday.

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

Answering a claim by some of Trump's supporters — and Trump himself — that the measure sends 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 — in a world that hungers for more 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 Wednesday.

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

“What the American people and the entire world will see from the debate we're about to have in the Senate is that there is abundant support for the United States taking tough positions with regard to Iran,'' Lee said Wednesday. "And as part of that we want to make sure that any military action that needs to be authorized is in fact properly authorized by Congress. That doesn't show weakness. That shows strength.''’

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,'' Kaine said. "An offensive war requires a congressional debate and vote.''

The principle of congressional approval is established for an important reason, 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.''

Trump disputed that, arguing in two tweets Wednesday that a vote against Kaine's proposal was important to national security and pointed to the Jan. 3 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!''

Tehran responded to the U.S. attack on Soleimani by launching missiles at two military bases in Iraq that house American troops. The attack caused traumatic brain injuries in at least 64 U.S. soldiers, the Pentagon said..

Democrats and Republicans alike criticized a briefing by the Trump administration shortly after the drone strike, saying U.S. officials offered vague information about a possible attack being planned by Iran but no substantial details.

Kaine has long pushed for action reasserting congressional power to declare war. At Republicans' request, he removed initial language that targeted Trump in favor of a generalized statement declaring that Congress has the sole power to declare war. The resolution also directs Trump to terminate use of military force against Iran or any part of its government without approval from Congress.

Sen. Susan Collins, R-Maine, a co-sponsor, called the resolution “much-needed and long overdue,'' adding that over the past decade, "Congress has too often abdicated its constitutional responsibility on authorizing the sustained use of military force.''

Senate Majority Leader Mitch McConnell and many other Republicans oppose the resolution, saying it would send the wrong message to U.S. allies.

The Democratic-controlled House passed a separate, nonbinding war powers resolution last month. If the Senate approves the Kaine measure, the House could take up the Senate resolution later this month, House Majority Leader Steny Hoyer said.

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