The SAVE America Act is heading for a bruising Senate fight after President Donald Trump made its passage a top priority and warned he could withhold endorsements from Republican holdouts.

The bill would require proof of American citizenship to register and vote in federal elections. The House has passed a third version of the measure, but it now faces the Senate’s 60-vote threshold.

Democrats have vowed a weeks-long filibuster. They argue the bill is a voter-suppression measure dressed up as election security, and they say existing laws already make non-citizen voting illegal.

Republicans counter that the measure is needed to restore public confidence in elections. Trump has framed it as a flagship priority and is pressuring reluctant members of his own party to fall in line.

A Reuters fact-check in September debunked a viral video that falsely claimed the Senate had already advanced the bill by 51 votes to 48. Tracking sites note that the Senate has so far failed to pass the legislation.

The fight is expected to consume weeks of Senate floor time, setting up one of the defining clashes of the midterm season.