WASHINGTON - The Senate approved the Republican-backed budget Thursday night, a major step forward for the GOP effort to enact tax cuts.

The budget's passage will allow the GOP to use a procedural maneuver to pass tax legislation through the Senate with 50 or more votes, removing the need for support from Democratic senators.

"Tonight, we completed the first step toward replacing our broken tax code . . . We have a once-in-a-lifetime opportunity to replace a failing tax code that holds Americans back with one that actually works for them," Senate Majority Leader Mitch McConnell, R-Ky., said following the 51-49 vote.

Sen. Rand Paul, R-Ky., who believes the budget ought to reduce the deficit, was the only Republican to vote against it.

The budget opens the door to expanding the federal deficit by $1.5 trillion over 10 years.

Tax cuts that have become Republicans' essential policy objective since the Senate failed to pass multiple bills to rewrite Obamacare. Approval of the budget is expected to help shore up ties between Senate GOP leaders and President Donald Trump, who is angry at Republicans' failure on health care and bent on Congress approving a tax-reform package by the end of the year.

At the same time, by agreeing to the massive tax cut, Senate Republicans have officially moved the party far away from its promised goal of ensuring that the tax plan would not add to the deficit. The White House and House Republicans had vowed that the tax cuts would be offset with new revenue from the elimination of certain deductions, but that is no longer the GOP's goal. Instead, they have abandoned longstanding party orthodoxy of deficit reduction and are seeking a political win after months of frustration on Capitol Hill.

"I applaud the Senate for passing a budget," Speaker Paul Ryan, R-Wis., said in a statement. "This action keeps us on track to enacting historic tax reform that will mean more jobs, fairer taxes, and bigger paychecks for American families. We want Americans to wake up in the new year with a new tax code, one that is simple and fair."

Political pressure is on the GOP leaders' side: Republicans cannot cut taxes without first passing the budget resolution, giving them a strong incentive to support it.

Trump had projected confidence about the Senate's ability to approve a budget.

"I think we have the votes for the budget, which will be phase one of our massive tax cuts and reform," Trump said during a meeting with Puerto Rican Gov. Ricardo Roselló. "But I think we'll be successful tonight with respect to the budget . . . I think we have the votes. And frankly, I think we have the votes for the tax cuts, which will follow fairly shortly thereafter."

The vote came after just over six hours of amendment votes in which Democrats sought to call attention to controversial aspects of the GOP tax plan.

Democrats had planned to focus on four key tax-reform topics intended to make Republicans cast politically awkward votes: tax cuts for the wealthy, tax increases for the middle class, reductions to Medicare and Medicaid spending, and increases to the budget deficit.

Sen. Heidi Heitkamp, D-N.D., proposed an amendment to prevent tax increases on people making less than $250,000 a year. The measure would have also required the Senate to approve a tax-reform bill with 60 votes rather than a simple majority. Senate Budget Committee Chairman Mike Enzi, R-Wyo., called this language a "poison pill," and the amendment was defeated 51-47.

Not all of the Democratic amendments were related to the tax plan, however. Sen. Maria Cantwell, D-Wash., offered language aimed at preventing oil and gas drilling in the Arctic National Wildlife Refuge. It failed 52-48.

Several Republican amendments were adopted with broad support. Sen. Jeff Flake, R-Ariz., proposed language to make the "American tax system simpler and fairer for all Americans," which passed 98-0. Sen. Marco Rubio, R-Fla., proposed an amendment in support of increasing the child tax credit, which passed by voice vote.

Paul, one of the most vocal GOP critics of the party's budget, proposed several measures to lower the deficit and one designed to make it easier for the upper chamber to repeal Obamacare. The amendments failed.

The GOP appeared to win enough votes to pass its budget Tuesday when Sen. John McCain, R-Ariz., threw his support behind the proposal, saying it would provide a "path forward on tax reform." The return of Sen. Thad Cochran, R-Miss., from a health-related absence added to leaders' confidence they have the votes for passage.

Republicans control 52 of the Senate's 100 seats, meaning they could lose two votes from their own party and still pass the budget. Without Cochran, Republicans would have been able to lose only one vote.

The final amendment offered a moment of levity after a long night of votes. Sens. David Perdue, R-Ga., and Sheldon Whitehouse, D-R.I., offered language that would eliminate future vote-a-ramas.

The amendment was approved by unanimous voice vote, with applause.

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The Washington Post's Damian Paletta, Karoun Demirjian and Anne Gearan contributed to this report.