With the final details of a GOP tax reform bill still in flux, the U.S. Senate voted Wednesday evening to begin debate on the Republican plan, setting up an expected final vote on the measure by the end of the week, as GOP leaders joined the President in promising to pass a sweeping package of tax cuts and reforms by the Christmas break.

“It’s a once in a generation opportunity,” said Sen. Rob Portman (R-OH) on the Senate floor. “The last time we reformed our tax code in any substantial way was 31 years ago.”

“Passing tax reform is the single most important thing we can do right now, to shift the economy into high gear,” said Senate Majority Leader Mitch McConnell.

The Senate vote on a motion to start debate on the bill fell right along party lines, as despite some concerns about the details, GOP Senators stuck together on the first test vote on tax reform.

“I think we’re going to get to common ground and get this bill passed,” said Sen. David Perdue (R-GA). “I don’t want to see this bill destroyed because of a pursuit for perfection.”

Republicans had one very good sign after that first Senate vote was over, as Sen. Lisa Murkowski (R-AK) expressed her strong support for the GOP plan, which includes a provision that would open up an area in the Alaska National Wildlife Refuge – ANWR – to oil and gas production, something long sought by Republicans.

“The bill before us has a number of features that are very attractive to Alaskans,” Murkowski said in a statement, seemingly securing a key vote in favor of tax reform.

Republicans can only afford to lose two votes in the Senate; a 50-50 tie could still be broken with the vote of Vice President Mike Pence, who had dinner Wednesday evening with some of those still on the fence.

But GOP Senators were still trying to hash out some of the details; some like Sen. Marco Rubio (R-FL) wanted a larger child tax credit, as Rubio joined Sen. Mike Lee (R-UT) in filing an amendment that would pay for that larger credit by trimming back the plan’s signature corporate tax cut.

“The corporate tax rate in the United States is 35 percent,” Rubio said on the Senate floor. “We would reduce that to 22 percent,” while the current GOP plan is at 20 percent, something that’s been a red line for President Donald Trump.

“There are a lot of good things in this tax bill, but we can make it better,” Rubio said.

As for the final details of the Senate tax reform plan, those still were not available as Washington, D.C. went to bed on Wednesday night; GOP leaders filed the 521 page Senate bill, but were still working out the last minute changes to the plan.

Democrats complained bitterly about the process, but with only 48 votes, they still need GOP defections to defeat the plan.

“It’s not what the American people want,” said Sen. Bill Nelson (D-FL). “It’s what large corporations want.”

“Let’s stop this bill,” said Sen. Michael Bennet (D-CO). “This bill doesn’t deserve to be on the Senate floor, it’s a disgrace.”

Democrats were putting together their own amendments to offer on the floor, but Republicans seemed ready to block them with a united GOP front.

“People really want to get to ‘yes’ on this,” said Sen. Jim Risch (R-ID).

And Republicans seem to be on the verge of doing that in the U.S. Senate as soon as Thursday night.