In the two weeks since Speaker John Boehner shocked Capitol Hill by announcing his resignation, McCarthy has burned up the phone lines and pitched himself in closed-door meetings as an agent of change willing to take on GOP leaders in the Senate and the White House. And he's dealt with the surprise of a friend challenging him for the top job.
Now the current majority leader is facing continuing resistance from a band of staunch conservatives eager to undermine his candidacy and exert their new position of strength.
McCarthy appears to have at least 125 votes locked down Thursday to win the GOP nomination to succeed John Boehner as House speaker. But the secret ballot vote will only kick off a three-week race to October 29, when the winner needs 218 votes of the full House. McCarthy, officials close to him say, has 200 private backers.
Having already lost the endorsement of the House Freedom Caucus, which backed long-shot candidate Daniel Webster of Florida, the 50-year-old California Republican will have to ensure that he doesn't lose any more defectors from some lawmakers who may be skittish about voting publicly for McCarthy after privately giving him their support.
Lawmakers heading home for next week's congressional recess are already bracing for what they might hear from voters.
"Do you want to go home and tell your constituents that you voted for Boehner's right hand man?" said Rep. Thomas Massie of Kentucky, a leading supporter of Webster's. "It's easier to do that in a private secret ballot behind closed doors than it is in public."
Those constituents may want to hear more from McCarthy or the other candidates -- Webster and Rep. Jason Chaffetz of Utah -- as well.
How McCarthy can win over the hard right is an open question. At a Tuesday night meeting with the three candidates for Speaker, House conservatives went through a list of changes they wanted the next speaker to make. These include alterations to the committee structure, promises to bring up certain bills and accept greater input from the rank-and-file.
But the next speaker will also instantly have to grapple with big questions over his plans to resolve major fiscal clashes imminently confronting Congress, a subject that has largely been skirted in his series of closed-door sessions with lawmakers.
Boehner, the White House, Senate Majority Leader Mitch McConnell and Democratic leaders have begun talks about putting together a fiscal package that could be acted upon this month. While the talks are at their nascent stages, Congress faces a major deadline to raise the national debt limit by Nov. 5 -- just days after a new speaker is elected.
In his discussions with the rank-and-file, McCarthy has given little insight into how he would address that situation, other than to say that he would listen to his caucus.
"Let's face it: we've got some enormous fiscal issues that will be decided between now and Dec. 11 that we haven't decided yet," said Rep. Chris Stewart of Utah.
"I have 18 town hall meetings over a period of a week," said Rep. Blake Farenthold, R-Texas. "Kevin is going to have to do a good job on radio and television and wherever he is to convince the folks that I'm going to be hearing from if I'm going to be behind him."
McCarthy spent Wednesday in an endless series of meetings and phone calls seeking to lock down support -- and by all accounts, he was received rather well. Even on Tuesday night when he met with the House Freedom Caucus and other conservative groups, conservatives praised him afterwards for his handling of hostile questions, including one over a bill renaming a courthouse that he refused to put on the floor.
"Without a doubt, I think he already has some conservative support," said Rep. Mark Meadows, the North Carolina Republican who offered a resolution this summer aimed at booting Boehner from the speakership. "To suggest there are no conservatives for Kevin would not be factual."
McCarthy has also had to spend time talking about his gaffe on Benghazi -- his interview last week with Fox News in which he implied the House's special committee on the 2012 attacks political in nature to harm Hillary Clinton's presidential campaign.
Three minute speeches
At Thursday's conference meeting, each of the three men who are running to succeed Boehner pitched themselves to conservatives in the caucus.
"All of them made pretty good pitches; there was nothing very emotional, nothing divisive," Rep. Peter King, R-New York, told reporters coming out of the meeting.
"Everyone concedes that Kevin is going to get a large majority, but the question is what happens on the floor," King added.
The longest of long shots, Webster, is trying to make the case to his fellow Republicans that he's a man who believes in a bottom-up approach, something he practiced as speaker of the Florida state House.
Chaffetz, who stunned McCarthy by announcing his run just hours after backing the California Republican for the job, planned to argue that he will let congressional committees do their work and not exert his own influence.
And McCarthy, who has served as Boehner's chief deputy after Rep. Eric Cantor's shocking primary loss in 2014, continues to make the case he is his"own man," relying heavily on his deep well of friendships throughout the conference and distances himself from Boehner.
"Relationships are one of the things that matters in this business," said Alabama Rep. Robert Aderholdt, a conservative member of the Republican Study Committee, who backs McCarthy.
While Rep. Raul Labrador, R-Idaho, said the conservatives plan to take their push for Webster "all the way to the floor," Virginia Rep. Dave Brat added: "I don't think we've committed to anything yet."
Conservatives who oppose McCarthy said that the GOP leader might never get there.
"At the end of the day, he may never get 218 and then we'll be looking at some compromise candidate," said Rep. Tim Huelskamp, R-Kansas.
But more moderate Republicans brushed off the drama of trying to reach 218 — dismissing the Freedom Caucus and McCarthy naysayers are "noise."
"There's just a lot of hue and cry and cymbals ... lots of noise signifying nothing," Florida Rep. Ileana Ros-Lehtinen said. "He will (get 218). He's supported, and it will manifest itself -- on the floor and in the Thursday vote."
Georgia Rep. Rob Woodall used a sports analogy to say he believes the party will put up a united front — adding he "absolutely" predicts the final floor vote will only require a single ballot.
"You can beat on each other in the locker room and you can argue about the plays that got called last week, but sooner or later you got to put on your helmet and walk out on the field, and no one, not a single constituent in anyone's district back home is going to be served by folks running their own play on game day," Woodall said.
Despite increasingly pitched challenges to his speakership, Boehner never needed a second ballot to retain his post.
After the public slap from the House Freedom Caucus, McCarthy picked up an endorsement late on Wednesday night from one of the GOP's favorite conservative figures, former Vice President Dick Cheney.
Calling McCarthy a "a good man and a strong leader" Cheney, who served in House Republican leadership in the 1980's, vouched for the California Republican.
"We are in one of the most difficult and challenging periods of our history. It is more important than ever that we have strong conservative leadership in the House of Representatives," Cheney said in a written statement.
For his part, McCarthy on Wednesday afternoon wasn't too concerned.
"It's a healthy debate," McCarthy said. "We'll have a vote on the floor and in conference tomorrow - just like going through any other primary - will be the power of ideas."