Sen. John McCain will back the Senate GOP's tax bill, increasing the plan's chances of clearing the Senate by Friday.
McCain was one of the last GOP senators who had not committed to supporting the proposal.
"After careful thought and consideration, I have decided to support the Senate tax reform bill," McCain said in a statement Thursday. "I believe this legislation, though far from perfect, would enhance American competitiveness, boost the economy, and provide long overdue tax relief for middle class families."
McCain's stance, as well as an expected "yes" vote from Sen. Lisa Murkowski, R-Alaska, leaves few GOP senators undecided. Republican leaders are tweaking the bill to ease concerns by other skeptical senators, including Sens. Bob Corker, R-Tenn.; James Lankford, R-Okla.; Jeff Flake, R-Ariz.; Susan Collins, R-Maine; Ron Johnson, R-Wis., and Steve Daines, R-Mont.
All Republican senators voted Wednesday to start debate on the bill.
Republicans can lose two votes and still pass the tax plan under special budget rules if all Democrats and independents oppose it. Vice President Mike Pence can cast a tie-breaking vote for a simple majority.
McCain, 81, of Arizona, bucked his party and drew venom from President Donald Trump when he opposed a Senate Obamacare repeal plan in September. Returning to the Senate that week following a brain cancer procedure, he criticized the Senate GOP's rushed process and called for a return to regular procedural order.
McCain said his concerns about regular order were satisfied during the tax process.
The senator also addressed the tax bill's repeal of the Obamacare individual mandate, which requires most Americans to have health insurance or pay a penalty. Scrapping the measure would lead to about 13 million more people uninsured, with average Obamacare premiums rising 10 percent, according to nonpartisan Congressional Budget Office estimates.
McCain called the individual mandate "an onerous tax that especially harms those from low-income brackets."
Most lower- and middle-income groups will eventually see a tax increase under the Senate plan, according to Joint Committee on Taxation estimates. Wealthier Americans will broadly get tax relief from the bill.
The Senate legislation only temporarily changes individual income tax rates but permanently cuts the corporate tax rate to 20 percent from 35 percent.