The Republican-held Senate passed a resolution 55-45 today to constrain President Donald Trump from initiating offensive military action against Iran, likely setting the stage for a White House veto. Eight Republicans joined forces with Democrats in a rebuke of Trump’s strike against Iranian Maj. Gen. Qasem Soleimani last month after a barrage of attacks on US forces and diplomatic facilities from Iran-backed Iraqi militias.

“With passage of this resolution, we sent a powerful message that we don’t support starting a war with Iran unless Congress votes that military action is necessary,” said Sen. Tim Kaine, D-Va., who introduced the resolution, in a statement after the vote.

The Republicans who bucked the White House to vote for the Kaine resolution are Lamar Alexander of Tennessee, Bill Cassidy of Louisiana, Susan Collins of Maine, Mike Lee of Utah, Jerry Moran of Kansas, Lisa Murkowski of Alaska, Rand Paul of Kentucky and Todd Young of Indiana.

Why it matters: Trump’s assassination of Soleimani has prompted Democrats to reassert Congress’ war-making authorities. Democrats argue that the Soleimani strike was illegal as it was not authorized by Congress. Most Republicans have sided with the Trump administration, which has claimed that Soleimani was planning an imminent attack on US assets in Iraq. The strike came after Iran had hit US interests in Iraq with a flurry of rocket attacks, leading to the death of a US contractor in late December. While the Trump administration has briefed Congress on the Soleimani strike behind closed doors, it has refused to publicize the evidence supporting its claim that the Quds Force leader was planning an imminent attack.