Story highlights

The Senate voted to start floor debate Wednesday on the GOP tax plan

It's unclear if Republicans have the votes for final passage

Washington CNN  — 

The Senate has passed a procedural vote on the tax bill that allows debate to begin on the measure with a final vote expected late Thursday or Friday.

The procedural vote passed 52-48.

As the vote was finishing, President Donald Trump said on Twitter, “This week, the Senate can join the House & take a strong stand for the Middle Class families who are the backbone of America. Together, we will give the American people a big, beautiful Christmas present-a massive tax cut that lets Americans keep more of their HARD-EARNED MONEY!”

After learning blistering lessons from their health care debacle, the GOP Senate has managed – so far – to hold their conference together and keep inching toward legislation that will extensively rewrite the country’s tax code for the first time in three decades.

But, behind-the-scenes negotiations continue and there is plenty that could go wrong. The party nearly skidded off schedule Tuesday over concerns from Sens. Bob Corker of Tennessee and Ron Johnson of Wisconsin. Corker wanted to ensure that tax bill would actually lead to economic growth and was able to secure an agreement in principle that would trigger some kind of automatic tax increase if the GOP’s anticipated growth didn’t materialize. Johnson, meanwhile, went into the GOP tax vote in the Budget Committee still frustrated about the way the tax bill treated tax rates for pass-through business income, but voted anyway to advance the proposal, saying that he wanted to keep the process going.

The jury is still out on if both men support the final bill that comes to the floor. Senators will now enter a lengthy debate and amendment process that could leave the final tax bill changed yet again from what it is now. In the end, Majority Leader Mitch McConnell can only afford to lose two votes. If he loses three, the bill won’t pass.

There are also still a host of lawmakers who haven’t definitively said if they’ll back the final bill. Sens. Susan Collins of Maine, Lisa Murkowski of Alaska and John McCain of Arizona are still wild cards even as Collins and Murkowski have signaled they are warming to the proposal in recent days.

“I feel like that we’re making progress,” Collins said. “I am not there yet, but we’re making progress.”

RELATED: Lisa Murkowski saved Obamacare. But here’s why she may not abandon Republicans on taxes

And Corker’s idea of establishing a mechanism to automatically increase taxes if the tax bill doesn’t generate enough growth has struck a nerve with some conservative senators who say they aren’t thrilled about such a proposal.

“I am not too keen on automatic tax increases,” Sen. John Kennedy, a Republican from Louisiana, told reporters, noting that he was listening to what the final proposal was. “Right now, my feeling about it is, I’d rather drink weed killer than vote for the thing.”

Other Republicans also voiced concerns over the trigger during a news conference Wednesday.

“I think that the concern is well-intentioned, but I don’t know that it’s well founded,” Sen. Thom Tillis, a Republican from North Carolina, said.

But disagreements over the trigger could set up a conundrum much like the one that leadership encountered during its health care showdown. Appeasing one side of the conference – in this case deficit hawks – could come at a cost to votes on another side of the conference. Although many members were still holding back on whether including the trigger would mean a loss of their vote.

Sen. David Perdue of Georgia described how he sees the bill: “I come from a world where you really don’t ever get 100% of what you want, and I don’t want to see this bill destroyed because of pursuit for perfection.”

This story has updated and will continue to update with new developments.

CNN’s Phil Mattingly and Ashley Killough contributed to this report.