A deal to provide further U.S. assistance to Ukraine by year-end appears to be increasingly out of reach for President Joe Biden. The impasse is deepening in Congress despite dire warnings from the White House about the consequences of inaction as Republicans insist on pairing the aid with changes to America’s immigration and border policies.

After the Democratic president said this past week he was willing to “make significant compromises on the border,” Republicans quickly revived demands that they had earlier set aside, hardening their positions and attempting to shift the negotiations to the right, according to a person familiar with the talks who was not authorized to publicly discuss them and spoke on condition of anonymity.

The latest proposal, from the lead GOP negotiator, Sen. James Lankford, R-Okla., came during a meeting with a core group of senators before they left Washington on Thursday afternoon. It could force the White House to consider ideas that many Democrats will seriously oppose, throwing new obstacles in the difficult negotiations.