WASHINGTON - The Senate failed to muster enough votes Friday to approve a measure that would have insisted President Donald Trump come to Congress before engaging militarily with Iran, except in the case of self-defense.

The measure, a proposed amendment to the annual defense bill, would have needed 60 votes to be included in the $750 billion authorization senators passed Thursday. It encountered significant opposition from Republicans, who argued that it was unnecessary - and potentially dangerous, in sending a message to Iran that Congress might undermine the commander in chief in the midst of an escalating conflict.

The vote was 50-40, with four Republicans joining Democrats to support the measure: Sens. Jerry Moran of Kansas, Susan Collins of Maine, Mike Lee of Utah and Rand Paul of Kentucky. There were 10 no-shows: Democrat Sen. Chris Coons of Delaware and nine Republicans: Sens. Marsha Blackburn of Tennessee, Roy Blunt of Missouri, Mike Braun of Indiana, Richard Burr of North Carolina, Kevin Cramer of North Dakota, James Inhofe of Oklahoma, Pat Roberts of Kansas, Mike Rounds of South Dakota and Rick Scott of Florida.

"The timing couldn't be worse," Sen. Marco Rubio, R-Fla., said Thursday, arguing that the amendment was "just not necessary" given the president's powers to direct the military. "The redundancy is actually damaging. . . . It's only going to reinforce this belief among some in the regime that they can go further than they can."

Those supporting the effort rejected the argument that the president would fully respect Congress' authority to declare war - or stick to the plan about when and where to respond to provocations with military force.

"They're trying to create excuses for why we should ignore the Constitution and open the door to war with Iran," Sen. Tom Udall, D-N.M., the amendment's author, said Thursday. "President Trump has said he was ten minutes away from doing just that."

The debate now moves to the Democratic-led House, where lawmakers are expected to include a similar restriction in their version of the defense bill. Debate begins on the House floor next month.

The Senate and the House will then have to combine the two versions of the defense bill. Iran is one of several matters - along with nuclear weapons investments, the diversion of military resources to the U.S.-Mexico border, and overall funding levels for the Pentagon - that promise to be contentious.

Sen. Tim Kaine, D-Va., who co-authored the Senate's Iran amendment, surmised that it stood a good chance of being included in the final product "because of the fact that we did get a majority here [in the Senate] and we're gonig to get a strong vote in the House."

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The Washington Post's Mike DeBonis and Emily Davies contributed to this report.