“In conversations with Republican colleagues, especially after the briefing last week, they were discouraged that the attitude that was being communicated to us was that Congress was an annoyance,” Mr. Kaine said. “After that, they came to me and we have been able to make some amendments.”
The resolution, which would give Mr. Trump a 30-day deadline to come to Congress for authorization for military action in Iran, would still need to be passed by the House. And it would be unlikely to overcome a veto from Mr. Trump.
Mr. Kaine introduced the measure, which invokes the War Powers Act of 1973, as a privileged joint resolution, which allows him to force a vote on the measure and win over the support of a simple majority of senators. With 45 Democrats in the Senate and two independents who routinely vote with them, Mr. Kaine needed just four Republicans to sign on.
The vote could come as early as next week, though it is unclear if the timing will be affected by the impeachment trial, which Senator Mitch McConnell of Kentucky, the majority leader, announced would begin next Tuesday. Democratic leaders will also need to corral senators running for president back to the Capitol to ensure passage.
The House passed similar legislation last week, though that measure was viewed as largely symbolic without the force of law.
But even Mr. Kaine’s legislation, considered to be the stronger of the two measures, has its limitations. The War Powers Resolution only restricts actions by the United States military, so it would not stop Mr. Trump from carrying out targeted attacks on Iranian military leaders or other discrete operations, as long as he carried them out covertly under the authority of the C.I.A.
Congress has rarely passed legislation invoking the War Powers Act in an attempt to restrain a president’s war-making authority. Last year, it sent to Mr. Trump just such a measure in a bid to cut off American military support of the Saudi-led campaign in Yemen, an intervention that has created the world’s greatest humanitarian disaster. While Mr. Trump vetoed that legislation, supporters of the legislation hoped it would create a new model for curtailing presidential war powers.