Last Wednesday, Senator John McCain revealed he's been diagnosed with a deadly and aggressive brain cancer after surgery on a blood clot in his brain revealed a malignant tumor. McCain's absence from Washington as he recovered from surgery was seen as an obstacle to Republicans' goal of moving forward on healthcare. Today, however, as Senate Majority Leader Mitch McConnell brings a motion to proceed vote to the floor of the Senate, John McCain is returning to Washington to cast what is seen as a pivotal vote.

McCain released a statement and tweet saying he was "looking forward" to returning to Washington.

With at least two Republican Senators having expressed opposition to moving forward on repeal, Senator McCain's vote is seen as a crucial one. But even as the healthcare vote looms later today, it's still unclear what bill will be voted on, only that McConnell will hold a procedural vote called a motion to proceed.

Both Democrats and Republicans railed against McConnell for keeping details of Tuesday's vote a secret.

On Face The Nation on Sunday, Republican Senator Susan Collins of Maine expressed her frustration with the lack of transparency in the process.

And Democrats did the same.

And some questioned whether it was right to force McCain to fly to DC just for this vote rather than allow him to continue to recuperate.

The U.S. Senate is scheduled to convene at 12pm ET on Tuesday.