The Arizona Republican 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. "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, all but erases potential GOP opposition to the plan. Republican leaders are currently tweaking the bill to appease 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.
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 tiebreaking vote for a simple majority.
McCain, 81, 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.
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