Susan Collins

Maine Sen. Susan Collins was one of the Republican senators to vote for a measure to end the war in Iran, a resolution that passed the Senate for the first time May 19 in a political rebuke to President Donald Trump.

The war powers resolution advanced 50 – 47. Along with Collins, Sens. Lisa Murkowski, R-Alaska; Rand Paul, R-Kentucky and Bill Cassidy, R-Louisiana, joined most Democrats to advance the measure. All New England senators voted for the measure.

The resolution faces an uphill battle to becoming law. But it's progress was a bad omen for the White House about potentially waning support in Congress for the war as rising gas prices, spiking inflation and the president's sinking polling numbers have become political liabilities for battleground GOP lawmakers ahead of the November midterm elections.

What is the war powers resolution?

The war powers resolution would restrain Trump’s ability to further attack Iran without congressional approval.

It would declare that Congress has the sole power to declare war and direct the removal of U.S. forces from Iran unless explicitly authorized by Congress.

Susan Collins one of several Republicans to defect

The legislative rebuke was a demonstration of how simmering tensions between the White House and Senate Republicans are boiling over amid Trump's efforts to push out several longtime Republican senators.

While Collins, Murkowski and Paul have voted for the war powers resolution in the past, Cassidy had opposed the measure in previous votes. His vote to advance the war powers resolution comes just days after he lost his primary reelection in large part because Trump crusaded against him.

And Sen. Thom Tillis, a retiring North Carolina Republican whom Trump has repeatedly publicly attacked, didn't vote at all. Neither did Sen. John Cornyn, a Texas Republican whom the president abruptly declined to endorse on Tuesday. (Republican Sen. Tommy Tuberville of Alabama didn't vote either.)

Will the War Powers Act for Iran become law

The resolution was introduced by Virginia Sen. Tim Kaine, D, on April 27.

The vote Tuesday was largely procedural and did not mean that the resolution passed the Senate. It faces a long and unlikely road ahead to actually become law. On May 14, the House narrowly rejected a similar resolution.

The last time such a vote succeeded in the Senate was after the American military invaded Venezuela, prompting five Republicans to buck Trump. Then the president posted negative comments about them on social media, and several reversed their votes days later.

Contributing: Zachary Schermele