Live Updates: Senate Nearing Final Vote on Climate and Tax Bill

Ms. Sinema had been the final holdout on the package after Senator Joe Manchin III, Democrat of West Virginia, struck a deal with top Democrats late last month that resurrected a plan that had appeared to have collapsed.

Her support brought Democrats closer to enacting the package and salvaging key pieces of their domestic agenda. It came just over a week after Mr. Manchin and Senator Chuck Schumer, Democrat of New York and the majority leader, stunned their colleagues with an agreement to include in the legislation hundreds of billions of dollars for climate and energy programs and tax increases, on top of a proposal to reduce the price of prescription drugs and to extend expanded health insurance subsidies.

“For over a year, I’ve made clear the importance of ensuring all legislation stays thoughtful and targeted to help our economy grow,” Ms. Sinema said in a statement on Saturday after a key procedural vote.

The final package, she added, “lowers costs for everyday Arizonans, makes health care more affordable and accessible, secures Arizona’s water and energy future, and creates jobs — and does so in a way that boosts Arizona’s innovation and economic competitiveness so Arizonans can build better lives for themselves and their families.”

With Republicans united in opposition, the measure needs the unanimous support of Democrats to move forward in the 50-50 Senate, so Democrats cannot afford even one defection.

Ms. Sinema had insisted on the removal of a provision that would have limited the preferential tax treatment of income earned by some wealthy hedge fund managers and private equity executives. Democrats instead added a new 1 percent excise tax that companies would have to pay on the amount of stock they repurchase.

That provision, Democrats said, was included to ensure that the package still reduces the federal deficit by as much as $300 billion, the same amount that Democrats aimed for with the original deal and a key priority for Mr. Manchin.

While most Democrats had been quick to rally around the deal that Mr. Manchin reached with Mr. Schumer when it was announced last month, Ms. Sinema had refused to weigh in and privately signaled that changes, particularly to the tax proposals, would be needed to win her vote.

An enigmatic centrist, Ms. Sinema had already forced her party to abandon its plans to overhaul much of the tax code, and her characteristic silence frustrated Democrats eager to take up the bill. After hearing that she had given the bill her support, several Democratic senators and aides celebrated, confident that final passage was within reach.

Alan Rappeport contributed reporting.