“It’s bringing Congress together in a way, frankly, I haven’t seen in my 12 years,” Senator Chris Coons, Democrat of Delaware and a confidant of President Biden, said on Tuesday of the consensus to support Ukraine. “You’d have to go back to 9/11 to see such a unified commitment.”

That has meant a retreat by both parties from the policy proposals and political messages that most thrill their core supporters. On the left, Democrats are acquiescing to higher military spending and dropping a bid to pull back rapidly from fossil fuels. On the right, Trump-era isolationism and attacks on the trans-Atlantic alliance are being relegated to the fringe in Congress. Plans to make the president’s son Hunter Biden and Ukrainian corruption front and center in a Republican-controlled House now seem far-fetched.

“It’s grounding,” said Representative Peter Meijer, Republican of Michigan, who was once an intelligence analyst in Afghanistan and now faces a challenger on his right endorsed by former President Donald J. Trump. “You can propose something that outstrips the bounds of reality, that’s fine. Then you look at what’s happening in Ukraine, and it’s like a car accident, a near-death experience that snaps you back to reality.”