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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.

McCain’s opposition to the 2001 and 2003 Bush tax cuts, combined with the partisan nature of the recent process and the stinging defeat he provided the party on health care, left top GOP aides to speculate he may eventually oppose the measure. McCain, who at one point spoke positively of the process, was less forthcoming in recent weeks, raising concerns he had shifted his view. Privately, two sources said, he voiced frustration over how much the bill had shifted during the Senate Finance Committee consideration.

But in the end, McCain chose to come on board. The support provides another boost for Republicans who, heading into the final hours of consideration, believe the tax proposal is on the brink of passage.

Perhaps more importantly, it gives Senate GOP leaders space should their efforts to assuage the concerns of other GOP senators fall short or prove to be too expensive. They can afford to lose two Republican votes. Now, it’s a certainty that McCain won’t be one of them.