On Monday night, Sen. John McCain said he would return to Washington to vote on key issues — most notably the Senate's push to overhaul the US healthcare system.

McCain's dramatic return a week after announcing his diagnosis of brain cancer seems to indicate that the vote on the Senate's healthcare push, expected on Tuesday, is extremely close for Republican leaders.

Senate Majority Leader Mitch McConnell said on the floor of the Senate on Tuesday that the body would take up a motion to proceed, a key procedural vote to begin debate on the healthcare bill, later in the day.

"Any senators who vote against starting debate is telling American you are just fine with the Obamacare nightmare," McConnell said.

Recent holdouts on previous healthcare proposals, including Sens. Rob Portman and Dean Heller, said they would vote for the motion. Heller, who in a press conference had come out against the original Senate bill, announced his decision in a statement but did not guarantee a "yes" vote on the final bill.

"If the final product isn't improved for the state of Nevada, then I will not vote for it," Heller said. "If it is improved, I will support it."

After a rush of confusion leading up to the vote, it appears McConnell's tactics are starting to come into view.

Based on statements from various senators, McConnell's current plan is to first allow a vote on a bill to repeal the Affordable Care Act, the healthcare law known as Obamacare — a vote that would most likely fail. Senators would then vote on the Better Care Reconciliation Act, which proposes repealing and replacing the ACA.

If the BCRA included two amendments, one from Sen. Ted Cruz and one from Portman, the bill would need 60 votes to pass and thus would most likely fail.

(If you want a full breakdown of the three major pathways and their potential effects, Business Insider's Lydia Ramsey has everything you need to know here.)

Then, it seems, McConnell would move on to a so-called skinny repeal, which would cobble together some amendments to repeal certain parts of the ACA, like the individual mandate and medical-device tax.

If the Senate passed this skinny bill, the thinking goes, lawmakers from the House and Senate could come together to work on a compromise bill in a conference committee. What that would look like is unknown, but it would at least advance the bill and give McConnell some sort of victory.

Despite this newly formed plan, few GOP senators knew before Tuesday what the details of the vote or the plan would be.

"If you don't know of those things before you go in, you're sort of voting in a blind fashion," Sen Rand Paul, a conservative-leaning member of the conference, said on Monday. "I think we need more information. CBO needs to have scored the whole bill."

McConnell then assured Paul that a repeal-only bill and a skinny repeal bill would come up, putting Paul in favor of the motion.

"I don't have a clue what we're gonna be voting on," Sen. Ron Johnson, a Republican from Wisconsin, said on Monday. "I just need to know what I'm going to vote on. I'm not real happy with the process."

It appears McConnell has the backing of President Donald Trump, who met on Monday with a moderate skeptic of the BCRA, Sen. Shelley Moore Capito, in her home state of West Virginia. On Tuesday, Trump launched into an early-morning tweetstorm urging senators to move forward with the process.

"Big day for HealthCare," Trump tweeted. "After 7 years of talking, we will soon see whether or not Republicans are willing to step up to the plate!"

When the Senate does "step up to the plate" on Tuesday, here's a rough outline of how the process is expected to go down:

McConnell will call for a motion to proceed on the House's American Health Care Act. Since every Democrat is expected to vote against this, no more than two GOP senators could do so. Sen. Susan Collins has said she will vote against it, leaving McConnell little room for error.

If the motion succeeds, 20 hours of debate — in legislative time — will begin, split equally between Democrats and Republicans.

The first amendment to be voted on is likely to be the Obamacare Repeal Reconciliation Act to satisfy Paul and other conservatives. This plan is likely to be shot down by moderates.

The first amendment to be offered procedurally — but the second to be voted on, the news website Axios reported — would be the BCRA, which was last updated on Thursday. Again, Republicans can afford only two defections.

According to reports, there is an agreement between Portman, a more moderate holdout, and Cruz, a conservative, on an amendment that would keep the structure of the BCRA but allow insurers to sell non-Obamacare-compliant policies and throw in $100 billion to the state stabilization fund. But since that would require 60 votes to pass, and it has not been scored by the Congressional Budget Office, it is almost certainly doomed, since there are only 52 Republicans in the Senate.

There could then be a series of amendments to the House bill, including those from Democrats. Additionally, other healthcare legislation could be slotted in for a vote.

Finally, McConnell will try to push the Senate to pass a bundle of smaller amendments focused on repealing aspects of the ACA, like the individual mandate and medical-device tax. After this, the House and the Senate would flesh out a full replacement bill in a conference committee.

According to a press release from McCain's office, the senator will be in the chambers to vote on the motion around 2:45 p.m. ET, indicating the motion will be brought to the floor around then.