McCain stressed his opposition wasn't personal but focused on Mulvaney's previous policy positions, accusing the conservative lawmaker of working to "torpedo" Senate efforts to increase defense spending. "My decision to oppose this nomination is not about one person. It is not about one Cabinet position," said McCain, the chairman of the Armed Services Committee. "This is not political. This is about principle." "[He] has spent his last six years pitting the national debt against our military," McCain said of Mulvaney.

"Voting in favor of Congressman Mulvaney’s nomination would be asking Secretary Mattis to spend less time fighting our enemies overseas and more time fighting inside-the-beltway budget battles with an OMB director," he said. McCain said he had "great reluctance" about opposing Mulvaney, saying that "under most circumstances, I always give the benefit of the doubt to the incoming president." McCain is the first GOP senator to say that he will oppose Mulvaney to be President Trump's budget chief. The Senate is expected to take a final vote on his nomination on Thursday. McCain argued that confirming Mulvaney would effectively pit the White House's budget office against the Pentagon.

McCain's position will give the GOP little room for error to get Mulvaney cleared through the Senate. Republicans have a 52-seat majority, and Cabinet nominees need a simple majority to be approved by the upper chamber.

With McCain voting no, Republicans can only lose one additional GOP senator.