The GOP’s $1.5tn tax reform bill is heading toward a final vote and possible victory before Christmas with support from key Republican holdouts who had raised concerns about the deficit and a child tax credit.

Bob Corker of Tennessee, a frequent critic of Donald Trump and the only Republican senator who voted against the legislation in the Senate, announced Friday that he would back a final version of the bill despite his concerns about the deficit. Corker said: “I believe that this once-in-a-generation opportunity to make US businesses domestically more productive and internationally more competitive is one we should not miss.”

Senator Marco Rubio, of Florida, also said he would back the bill, a day after he threatened to vote against it if lawmakers did not expand access to a child tax credit for lower-income families. He tweeted that the legislation would allow a higher percent of a child tax credit to be refundable, calling it a “solid step toward broader reforms which are both Pro-Growth and Pro-Worker”.

Marco Rubio (@marcorubio) 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.

The backing of Rubio and Corker means the legislation, a signature policy initiative of Trump, is on track to pass. The news came days after Republican control of the Senate shifted to a tight 51 to 49 margin following the election of Democrat Doug Jones over Republican Roy Moore in the closely watched Alabama Senate race.

The Democrats have called on Jones to be immediately seated so he can vote on the tax bill.

The controversial legislation was rushed through the Senate early this month after lawmakers received a nearly 500-page document, some of it handwritten, hours before the vote. The legislation represents the most sweeping overhaul of the tax code in decades and stands to benefit big businesses and the rich.

Critics have called the legislation a handout for the super-rich that would hurt lower-income and middle-class families. Bernie Sanders labeled it “one of the greatest robberies in American history”.

Republicans merged the House and Senate versions of the legislation and released a final bill late Friday, with the goal of voting before taking a recess for the rest of the year. Majority leader Mitch McConnell said the Senate would “get this bill done” next week.

Opponents have said the legislation would further exacerbate inequality in the US, and the latest Republican agreement seeks to gut a key part of Barack Obama’s healthcare law. The bill would repeal the “individual mandate”, which required people to purchase health insurance or pay a fine. Millions could be left without insurance and premiums could rise as a result.

Trump, who has yet to score a single major legislative victory, has called on Republicans to send a bill to his desk by Christmas.

“We want to give you, the American people, a giant tax cut for Christmas. And when I say giant, I mean giant,” Trump said in a speech at the White House on Wednesday.

The final bill would also open the Arctic national wildlife refuge in Alaska to oil and gas drilling, a proposal that has drawn fierce backlash from environmentalists and tribal groups.

The president has expressed frustration with the lack of movement on his agenda on Capitol Hill, and was left bruised when repeated attempts by Republicans to repeal and replace Obama’s healthcare law fell short this summer. Republicans, who control both chambers of Congress, have expressed confidence they will not suffer similar embarrassment with tax reform.

The final bill is poised to lower the top tax rate for families and individuals to 37% from 39.6%. It will also lower the top corporate tax rate to 21%, one percentage point higher than the 20% Trump had called for.

White House press secretary Sarah Huckabee Sanders praised Corker’s reversal in a statement Friday. She said: “He sees a great entrepreneurial spirit being released in our country and he is a part of that spirit. When these massive tax cuts and incentives kick in, jobs and growth will follow at a very high level.”

The Joint Committee on Taxation and the Congressional Budget Office, both nonpartisan research groups in Congress, have projected the Republican proposals would disproportionately benefit corporations and wealthy people.

Republicans in the Senate are using a process known as “budget reconciliation” that enables them to pass the tax plan with a simple-majority vote. As a result, they can only afford to lose two votes under their current two-seat majority – a scenario in which vice-president Mike Pence would act as the tie-breaking vote.

If Republicans don’t advance their plan before Jones is sworn in, they would have just one vote to lose.

“This is what the American people have been waiting for: more jobs, fairer taxes, and bigger paychecks,” House speaker Paul Ryan said in a statement Friday night, saying the bill was “now only two votes and a signature away from becoming the law of the land”.