(CNN) Republicans have finished writing the final version of their tax bill, clearing the way for a floor vote that leaders say they are optimistic will pass -- and at least two key Senate votes have gone from "no" to "yes."

Sen. Marco Rubio, a Republican of Florida, is a "yes" on the tax reform bill, per a source close to the senator.

In two tweets earlier Friday afternoon, Rubio said, "For far too long, Washington has ignored and left behind the American working class. Increasing the refundability of the Child Tax Credit from 55% to 70% is a solid step toward broader reforms which are both Pro-Growth and Pro-Worker. ... But there is still much more to do in the months and years to come. The progress made on the Child Tax Credit would not have been possible without the support of @SenMikeLee, @SenatorTimScott, and @IvankaTrump."

Rubio and Lee clearly got more money toward refundability of the child tax credit, expanding that part to $1,400, from $1,100. The expansion of the refundability piece, which is part of the $2,000 child tax credit in the tax bill, was made explicitly to secure Rubio's support.

In addition to Rubio, Sen. Bob Corker, who initially voted against the bill when it first passed the chamber, announced Friday afternoon he'd back the bill.

"After many conversations over the past several days with individuals from both sides of the aisle across Tennessee and around the country -- including business owners, farmers, chambers of commerce and economic development leaders -- I have decided to support the tax reform package we will vote on next week," the Tennessee Republican said in his statement.

As for locking up the remaining votes, congressional negotiators appeared optimistic.

"I'm confident at the end of the day, the Senate will approve this conference committee report," House Ways and Means Chairman Kevin Brady told reporters Friday.

Sen. Rob Portman, a Republican from Ohio and member of the tax conference, said "I'm confident we'll have the votes."

"It's a great bill and it's closed now. I think we'll get the support on Tuesday or Wednesday and have the vote. We already had enough senators as it came through the Senate and the bill has even improved further in the House-Senate conference process. I'm confident we'll have the votes," Portman said.

The language is set

The language was finalized late Thursday night and GOP leaders had to make 100% sure they had the votes, because after noon Friday, there could no longer be any changes. The bill text will be released Friday at 5:30 p.m.

Once the conferees sign the report, as they were scheduled to do this morning -- it's locked in. There are no last minute deals, or amendments, or fixes, or tweaks. That's it.

Keep in mind: "It's fair to say we wouldn't lock ourselves into something if we didn't think we had the votes," one GOP aide working on the bill told CNN late Thursday night.

But Senate leaders were scrambling throughout the day and as of now, the votes of at least four senators, and the health of two others, aren't publicly locked in.

The timeline

The exact timeline has grown increasingly fluid, both because of the potential for absent Republican senators, but also for procedural reasons.

As it stands right now, it looks like the House will go first -- this is important because moving it through the chamber actually helps Republicans limit Senate Democratic procedural hurdles or roadblocks when it travels across the Capitol.

But starting in the House risks the possibility of moving forward on something not entirely scrubbed for Senate rules (they're in the process of doing this right now), and House Democrats also have tools to make the process painful or potentially problematic.

Bottom line: The House is likely to go first, with a vote on Tuesday. Senate would then follow, likely starting work on Tuesday and finishing up as soon as Tuesday night or early Wednesday. Again, this all remains subject to change.

The rationale

Low approval ratings. A plan that opens them up to new Democratic attacks. Extremely fast process.

Several Republican lawmakers have acknowledged to CNN that the final process is ripe for potential errors just due to trying to cram super important, super complex legislation through a convoluted rule and negotiations process in a matter of weeks -- or days, or hours.

So why are Republican actually doing this?

This, in talking to dozens of GOP lawmakers and aides, is a question that frustrates them to no end. Here's what they say:

They believe in the policy -- yes, it's weighted heavily toward the corporate side, but they believe in their pitch of increased growth and wages that will create more for everyone else. They believe the poor approval ratings are because of what they perceive as disinformation and attacks. Take that for whatever it's worth, but that's been repeatedly stated by top Republicans in recent days. The political imperative is so, so real here. The idea of doing something, anything, before the end of the year has been the driving force of this process day in and day out for weeks. "You can't overstate that," one senior Republican aide told CNN. "We promised we'd do this. We're now doing it."

How this whole thing is paid for: That's a great question! And the lack of details on this are telling in two regards:

This is by far the hardest part of the process. Sure, it's easy to give people the things they want that cost money. But it sure is difficult to take things away to pay for them. The revenue pieces of this are overwhelmingly unpopular -- raising certain rates, sunsetting other things, phasing out provisions, eliminating or capping various deductions, etc. They upset interests, and advocates, and the lawmakers themselves.

Here's what we know -- Republicans raised the corporate rate by a point. The likely raised the repatriation rate as well, I'm told. There were talks of moving up the sunset of the individual rates, which was set at the end of 2025 in the Senate bill, to 2024. None of these are politically popular options. But some, if not all, will be needed to pay for the changes made.

The votes besides Rubio and Lee

Sen. Susan Collins: The public statements of Collins have been fairly bullish on the bill. She hasn't committed yet -- she never does until she sees final text, but she's made clear she believes the deals she made with Senate Majority Leader Mitch McConnell the first time around will stand. What does that mean? Look, the skepticism about whether Collins can actually get what she said McConnell would give her is both thick and snark-filled. That said, she wouldn't be where she is publicly if she wasn't comfortable with what she's been told privately. And several aides told CNN the private assurances are real. That's what explains why leaders feel like she'll be there in the end.

Sen. Jeff Flake: The expectation remains that Flake will come around. Again, the issue he's always been set on is the expensing provision. As of early this morning, it's still unclear where that -- whether it sunsets, which Flake hated, or gradually phases out, which got him on board -- remains.

The health issues:

Sen. John McCain: McCain is unknown right now. Senators on Thursday said they all thought he would be back, but acknowledged this latest McCain is unknown right now. Senators on Thursday said they all thought he would be back, but acknowledged this latest round of treatment has been particularly taxing on him

Sen. Thad Cochran: Cochran's spokesman told CNN's Sunlen Serfaty on Friday that despite a "more extensive" procedure this week that left the Mississippi senator looking a bit beat up, "He is still set to be available to vote when need be."

Diving deeper on Rubio

This issue is not new -- and Rubio's position on it (and frustration with GOP leaders about it) isn't either. What is new is the red line he drew Thursday, the type of play he's been reluctant to make throughout this debate. But it's one that was clearly driven by frustration. GOP leaders during the Senate debate said they simply couldn't find the money for his proposal. Then they found the money for all sorts of other things. That, to understate this, did not please the senator from Florida.

But it's also important to note that many Republicans don't agree with Rubio on the policy -- they view refundability (the amount someone is eligible to receive beyond their income tax liability) as a form of welfare. So there's conservative policy rationale for pushing back against the idea. Rubio counters very clearly that adding money for those with children, even if they pay no income tax at all, is a worthwhile expenditure of paramount importance given the economic hardship they face. It's not a new dispute inside the party, but it is one that has certainly flared publicly to a degree hasn't existed outside of think tanks and policy shops.

That said, Rubio, after negotiations again fell short of his expectations Wednesday, exercised his maximum point of leverage Thursday. It clearly worked to some degree -- GOP negotiators went back to the table with him. To what degree? Well, we'll see Friday.

The Bite of the Byrd rule:

We've noted the restrictive Senate rules many times, and last night officially took down another if not essential, but at least somewhat coveted, GOP issue -- the repeal of the Johnson amendment. We've noted that this was likely to happen several times -- it's the prime reason it wasn't in the Senate bill to begin with (it was in the House version.)

Good time to remind folks: This is why the Senate originally wanted to go first. The Johnson amendment isn't an issue that's fatal or hugely problematic to the bill itself -- Republicans wanted it in there, but it wasn't a central piece. But that might not be the case with other provisions.

As the Johnson amendment demonstrates, they have clearly been going through the Byrd bath process (this all takes place behind the scenes) to ensure the conference report doesn't run afoul of anything. But it's always a bit of a high wire act.

This story has been updated with additional developments and will be updated with more developments.