The G.O.P. abandoned a bipartisan border security bill that also aided Ukraine after Democrats called their bluff on immigration, agreeing to tough measures Republicans demanded.

Congressional Republicans thought they had set a clever trap for Democrats that would accomplish complementary political and policy goals.

Their idea was to tie approval of military assistance to Ukraine to tough border security demands that Democrats would never accept, allowing Republicans to block the money for Kyiv that many of them oppose while simultaneously enabling them to pound Democrats for refusing to halt a surge of migrants at the border. It was to be a win-win headed into November’s elections.

But Democrats tripped them up by offering substantial — almost unheard-of — concessions on immigration policy without insisting on much in return. Now it is Republicans who are rapidly abandoning a compromise that gave them much of what they wanted, leaving aid to Ukraine in deep jeopardy, border policy in turmoil and Congress again flailing as multiple crises at home and abroad go without attention because of a legislative stalemate.

The turn of events led to a remarkable Capitol Hill spectacle this week as a parade of Senate Republicans almost instantly repudiated a major piece of legislation they had spent months demanding as part of any agreement to provide more help to a beleaguered Ukraine. Even Senator Mitch McConnell of Kentucky, the minority leader and foremost Republican advocate of helping Ukraine, and Senator James Lankford, the Oklahoma Republican who invested months in cutting the border deal, suggested they would vote to block it on the floor in a test vote set for Wednesday.

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