Speaker Mike Johnson on Wednesday dug in against President Biden’s efforts to revive stalled legislation to send aid to Ukraine, saying the Republican-led House would not entertain it unless Democrats agreed to a far more severe crackdown at the U.S.-Mexico border than they have been willing to consider.

Mr. Johnson’s latest ultimatum cast further doubt on the prospects of Congress’s approving Mr. Biden’s request for tens of billions of dollars in emergency security assistance to help Ukraine fight off Russia.

Republicans have insisted that such a package be paired with measures to clamp down on migration at the southwestern border, and a bipartisan group of senators has made considerable progress on striking a compromise to do so. But the plan has no path to enactment if the House, where a sizable far-right contingent is pressing for even tougher immigration policies, refuses to accept it.

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