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.