Sen. John McCainJohn Sidney McCainThe Hill's 12:30 Report Senate gets to work in August — but many don’t show up Rand Paul’s Russia visit displays advancement of peace through diplomacy MORE (R-Ariz.) announced Wednesday that he will not support Rep. Mick Mulvaney's (R-S.C.) nomination to lead the Office of Management and Budget (OMB).
 
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.
 
McCain argued that confirming Mulvaney would effectively pit the White House's budget office against the Pentagon. 
 
"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'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. 
  
Still, it appears unlikely that Democrats will be able to block Mulvaney. Two GOP senators who had been on the fence appeared to throw their support behind Mulvaney on Wednesday. 
 
Sen. Susan Collins (R-Maine) said she had talked with him during a closed-door meeting about the difference between being OMB director and a House member from a conservative district. 
 
"I talked to him about his positions on defense spending, the fact that he has never voted for a budget or debt limit increase," she said.
 
Aides for Sen. Thad CochranWilliam (Thad) Thad CochranGOP Senate candidate doubles down on Robert E. Lee despite Twitter poll GOP Senate candidate polls followers on whether Robert E. Lee was hero or villain Mississippi courthouse named for Thad Cochran MORE, the chairman of the Appropriations Committee, also told the Associated Press and CNN that the Mississippi Republican would support Mulvaney. 
 
Updated: 6:06 p.m.