Speaker Mike Johnson warned Friday that an emerging Senate border deal — if reached — would be “dead on arrival” in his chamber, delivering the latest blow to the national security supplemental’s prospects.

The “Dear Colleague” letter caps off a week that laid bare the divisions among Republicans that threaten to tank whatever bipartisan plan Senate negotiators — who have vowed to work through the weekend to try to finalize an agreement — may eventually produce on U.S.-Mexico border policy changes and related funding.

In recent days, as senators have inched closer to a border compromise that’s key to unlocking billions of dollars of aid for Ukraine, Israel and the Indo-Pacific, the path for broader national security supplemental funding has only gotten harder.

Former President Donald Trump has loomed over the talks, openly lobbying for GOP lawmakers to accept nothing less than a “perfect” solution to the influx of migrants at the U.S. southern border. The pressure shaped a closed-door Senate Republican gathering Wednesday evening, in which Minority Leader Mitch McConnell, R-Ky., acknowledged the presumptive GOP nominee’s influence over the process as the 2024 election season heats up.

But Johnson, R-La., threw up another roadblock in his Friday letter, in which he declared Senate negotiations dead, reiterated House GOP support for a House-passed immigration bill and pledged to move forward with the impeachment of Homeland Security Secretary Alejandro Mayorkas in the coming week.

To Johnson and other House Republicans, future Ukraine support is contingent on the inclusion of what the GOP leader called “core legislative reforms” from the House-passed immigration bill that aims to restrict migrants’ asylum eligibility, reinstate family detention and more — a prospect that’s a nonstarter for Democrats.

The lead Senate Democratic negotiator, Sen. Christopher S. Murphy of Connecticut, said this week that bipartisan border talks have produced the outline of an agreement, with most issues resolved although legislative text wasn’t yet ready. Appropriators in the chamber have begun assessing the cost of parts of the draft language, lawmakers said in recent days.

Meanwhile, lawmakers on both sides of the aisle vowed to continue discussions, though they acknowledged they’ve reached “a critical moment,” in the words of Sen. John Thune of South Dakota, the chamber’s No. 2 Republican.

“Now as we’ve seen over the past day, getting a bipartisan agreement on the supplemental is very difficult but I am glad that now negotiations are continuing to move forward,” Senate Majority Leader Charles E. Schumer, D-N.Y., said on the floor Thursday. “Of course there are still issues that must be settled, but negotiators will work all weekend in an effort to get this done.”

Johnson in his letter characterized the Senate as seemingly “unable to reach agreement.”

“If rumors about the contents of the draft proposal are true, it would have been dead on arrival in the House anyway,” he wrote to his fellow House Republicans.

The path forward for the national security supplemental was further complicated by allegations that a dozen United Nations Relief and Works Agency staff members may have been involved in Hamas’ surprise assault on Israel Oct. 7. The State Department paused additional funding for UNRWA as it reviews those allegations, an agency spokesman said in a Friday statement.

The House Foreign Affairs Committee’s oversight and accountability panel is holding a hearing on UNRWA next week, with the allegations likely to play a major role.

Aidan Quigley contributed to this report.