The office of Sen. John McCain (R-Ariz.) said late Monday he would return to the Senate this week ahead of a vote to debate a bill that would gut Obamacare and potentially deprive tens of millions of Americans of health insurance.

“Senator McCain looks forward to returning to the United States Senate tomorrow to continue working on important legislation, including health care reform, the National Defense Authorization Act, and new sanctions on Russia, Iran and North Korea,” a statement from his office said.

Look forward to returning to Senate tomorrow to continue work on health care reform, defense bill & #RussiaSanctions https://t.co/VQBtovnwF1 — John McCain (@SenJohnMcCain) July 25, 2017

The date of McCain’s return was initially unclear after he announced last week he had been diagnosed with brain cancer. Doctors told the 80-year-old that they found a brain tumor known as a glioblastoma during surgery earlier this month to remove a blood clot above his left eye.

Several senators had expressed hope that McCain, a critical vote in any Republican plan to debate health care repeal, would return in time to cast it. Senate Majority Whip John Cornyn (R-Texas) reportedly said he “personally volunteered to rent an RV” to drive his colleague to Washington, according to Politico.

The Senate is set to vote Tuesday to begin debate on legislation related to repealing the Affordable Care Act ― but no one knows what’s in the bill.

Majority Leader Mitch McConnell (R-Ky.) plans to ask colleagues to vote for a procedural motion on a health care bill the House passed in May. If such an effort is successful, any number of amendments could be added to that bill that could make it function as anything from a full repeal of the ACA, to the GOP’s long-favored repeal-and-replace plan. But no one, including the senators expected to vote on Tuesday, is sure what McConnell is planning.

“We are potentially one or two days away from a vote on a bill that would reorganize one-sixth of the American economy, impacting tens of millions of American lives, and no one knows what it is,” Senate Minority Leader Chuck Schumer (D-N.Y.), told the New York Times.

McCain’s presence at any vote has been seen as crucial after McConnell failed to pass early versions of his health care plans when several Republican senators voiced their opposition to the bills. The party holds just a 52-seat majority in the chamber, and Sen. Susan Collins (R-Maine) is already likely to vote against the motion to debate the legislation.

Though McCain is returning, it’s unclear if he will support any final health bill. Some have speculated that, as his office noted in its statement, he is returning to help shepherd the next National Defense Authorization Act through Congress.