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.

“This nomination is dead-even right now,” Patty Murray of Washington, the ranking Democrat on the Education Committee and de facto leader of the anti-DeVos movement, said Monday on the Senate floor.

Of public education, Murray said DeVos “doesn’t value it. She is someone who has dedicated her career and her inherited fortune to privatizing public schools, to tearing down public education — to defunding it in order to push more taxpayer dollars into private schools and for-profit charters.”

After the vote on DeVos, Senate Majority Leader Mitch McConnell, R-Kentucky, has set up votes this week for attorney general nominee Jeff Sessions, Health and Human Services nominee Tom Price and Steven Mnuchin, chosen to be treasury secretary, in that order. Democrats as a bloc have opposed each, but they are likely to be confirmed no later than Saturday if McConnell keeps the chamber in over the weekend.

As Democrats attempted to drag out the nomination process, Republican Whip John Cornyn, R-Texas, decried what he said amounted to “obstructionism and slow-walking” by Democrats and Minority Leader Chuck Schumer of New York over Trump’s nominees.

“All they can do, which is all they have done to this point, is to slow the process down for no reason other than the fact that they can,” Cornyn said. “It’s a strategy in search of a goal. They don’t have any particular goal in mind because at the end of the day, the president will get the Cabinet that he has nominated and deserves.”

Democrats say DeVos’s confirmation hearing cast doubts on her commitment to ensuring access to education for students with disabilities and showed her bias toward private schools, religious institutions and charters to the detriment of public schools.

Sen. Chris Murphy, D-Connecticut, said early Tuesday morning that the vouchers for private schools that DeVos champions have “become a means of both economic and racial segregation” because only wealthier and often white families can afford to supplement the public funds so their kids can get into private schools.

Lawmakers also criticized her complex financial holdings. Sen. Elizabeth Warren said DeVos’s only experience with student loans has been her own investments in student-debt collections.

“It’s difficult to imagine a worse choice to head the Department of Education,” the Massachusetts Democrat said Monday on the Senate floor. “We need someone in charge of the nation’s schools who knows what they are doing and puts young people first.”

The fight over the confirmation took on new life last week when two Republicans, Susan Collins of Maine and Lisa Murkowski of Alaska, said they planned to vote against DeVos when the nomination went to the floor. Both said DeVos doesn’t understand rural public schools enough to calm their concerns.

Since then, lawmakers in both parties have said their offices were flooded with calls and letters from constituents opposing DeVos’s elevation. Sen. Tim Kaine, D-Virginia, said his office has received more contacts on this issue than any other since he was sworn into the Senate in 2013, including during the 13-day government shutdown that same year.

Constituents “want someone committed to public schools, someone knowledgeable about the federal role in education,” said Sen. Jack Reed, D-Rhode Island. “They have determined that Betsy DeVos is not that person.”

On Monday, Democratic senators and union officials held a rally against her nomination outside the Capitol with several hundred protesters. The speakers said she doesn’t understand education for those with disabilities and would roll back Title IX sex-assault protections.

“Betsy DeVos wants to put a ‘For Sale’ sign in every public school in America,” Sen. Ed Markey, D-Massachusetts, told the crowd. “We will not let that happen.”

DeVos is a longtime Republican Party donor and activist. She was appointed in 2004 by then-President George W. Bush to the board of the Kennedy Center for the Performing Arts in Washington. She and her husband, Dick DeVos, president of the Windquest Group, donated $22 million to the center’s program, which carries their name, to train managers and boards of arts institutions.

Dick and Betsy DeVos donated $3.2 million to Republican candidates in the 2016 election cycle, according to the nonpartisan Center for Responsive Politics.