If senators took their oaths seriously, votes to assert Congress’s authority in matters of war and peace would be unanimous.

But, of course, in today’s Washington oaths are often disregarded. As presidents have grown increasingly imperial, the Senate has too frequently abandoned its checking and balancing role, and, since Donald Trump took office, the vast majority of Republican senators have served as this particular president’s Praetorian Guard.

So it was significant that eight Senate Republicans joined Senate Democrats on Thursday in voting for legislation crafted by Senator Tim Kaine of Virginia to limit the president’s ability to order an attack on Iran.

Concerned about Trump’s erratic approach to the Middle East and about the prospect that similar tensions might at any point escalate toward war, Senators 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 broke with Trump to curb the president’s ability to wage war on Iran without express approval from Congress.

“For nearly two decades, Congress has been AWOL on certain matters of national security and attempted to pass the buck to our commander in chief when things go wrong,” said Young, a conservative who served as a US Marine and who usually sides with Trump. “It’s time for us to do our job.”

As he had done with considerably more success in this month’s impeachment trial, the president tried to rally Republican senators with a tweet declaring, “It is very important for our Country’s SECURITY that the United States Senate not vote for the Iran War Powers Resolution. We are doing very well with Iran and this is not the time to show weakness.” Senate majority leader Mitch McConnell echoed the president’s line, decrying “ill-conceived potshots at presidential authority.”

But Mike Lee, the Utah Republican who worked closely with Senator Bernie Sanders to cut US support for the Saudi Arabian military’s assault on Yemen, explicitly rejected the president’s call on senators to abdicate their constitutional responsibility. “We don’t send a message of weakness when we stand up for the rule of law,” announced Lee. “That’s a message of strength.”

House Speaker Nancy Pelosi plans to bring the Senate proposal up for consideration in the House, where it is all but certain to pass. Kaine’s plan would require the president to end hostilities targeting Iran within 30 days unless those hostilities have been approved by Congress. Trump’s expected to veto the measure. But that does not render this initiative irrelevant.

With a 55-45 bipartisan vote, the chamber has positioned itself in line with the majority of Americans. As Senator Tammy Baldwin explained, “The American people are sick and tired of sending young men and women to war in the Middle East.”

Baldwin, a cosponsor of the Kaine measure, argued Wednesday, “The Constitution is very clear that only Congress has the authority to declare war and I joined Democrats and Republicans to ensure that President Trump comes to Congress first before pursuing any military action against Iran and starting another war in the Middle East. Democrats and Republicans agree. We can’t let the President send more young men and women to war and repeat the mistakes of the past.”

That’s a big deal, said Trita Parsi, of the Quincy Institute for Responsible Statecraft. “True, Trump will veto it and we don’t have the vote to override it. But,” said Parsi, “it shows that a majority reject [Secretary of State Mike] Pompeo’s war of choice and that is a powerful constraint on Trump.”

Erica Fein, the advocacy director for the group Win Without War, called the Senate vote “a critical step toward not only blocking a potential Trump-led war with Iran, but toward reasserting its sole constitutional power to determine when, where, and whether the United States goes to war.” This, she argues, begins a process of renewing congressional authority that has the potential to tip the balance against war. “With Congress’s continued leadership, backed by an organized and activated grass roots,” she says, “we can be the necessary counterweight to Trump’s pro-war foreign policy.”

But Congress has to play its role. As Kaine says, “An offensive war requires a congressional debate and vote. This should not be a controversial proposition.”