The Democratic-controlled House passed a separate, nonbinding war powers resolution last month. The House could take up the Senate resolution later this month, said House Majority Leader Steny Hoyer, D-Maryland.
Two-thirds votes in the House and GOP-run Senate would be needed to override an expected Trump veto.
Answering a claim by some of Trump's supporters and Trump himself 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 — 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.
Sen. Mike Lee, R-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.’”
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. The number, according to some reports, might be more than 100.
Democrats and Republicans 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.” She said during the last decade, “Congress has too often abdicated its constitutional responsibility on authorizing the sustained use of military force.”
Senate Majority Leader Mitch McConnell, R-Kentucky, and many other Republicans opposed the resolution, saying it weakens the president and that it would send the wrong message to Iran and U.S. allies.