Vice President Mike Pence could make history Tuesday by breaking an expected tie in the Senate to confirm Betsy DeVos as U.S. education secretary, barring a last-minute defection by a single additional Republican to kill the nomination outright.

The nail-biter is the unexpected first test of Democrats’ ability to thwart President Donald Trump’s agenda, gathering momentum after two Senate Republicans said they wouldn’t support DeVos, a longtime GOP donor. The prospect of a 50-50 tie could make Pence the first vice president in U.S. history to cast a tie-breaking vote on a Cabinet post.

As Democrats grind the Senate to a near-halt by stalling all of Trump’s Cabinet picks still awaiting confirmation, DeVos has generated the most heat in the 52-48 chamber.

On Monday, Democrats began a campaign to speak for 24 straight hours in the hope of pressuring another Republican to vote against her.