Story highlights McCain had remained a wild card and had kept his position on the tax bill unclear even from leadership

For GOP leaders, McCain's decision to support the measure is equal parts major victory and relief

(CNN) Sen. John McCain said Thursday he will support Senate Republicans' tax plan, a major sign of progress for GOP leaders as the party barrels toward a vote on their overhaul of the US tax system by the end of the week.

McCain, who had remained a wild card and had kept his position on the tax bill unclear even from leadership, said that he came to support the legislation because he believed it had gone through committee and would improve the economic outcome of Americans.

"After careful thought and consideration, I have decided to support the Senate tax reform bill. 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 said in a released statement Thursday morning. "For months, I have called for a return to regular order, and I am pleased that this important bill was considered through the normal legislative processes, with several hearings and a thorough mark-up in the Senate Finance Committee during which more than 350 amendments were filed and 69 received a vote."

Republicans have 52 members in the Senate, and could only afford to lose two votes and still pass their legislation.

For GOP leaders, McCain's decision to support the measure is equal parts major victory and relief. One senior GOP aide said earlier this week his position was "a black box," and members and staff were consciously giving him space to come to a final decision.

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