Republican governors had arguably the best showing of anyone on Election Day last month, and a key mastermind behind their efforts is Phil Cox, who cut his teeth in Virginia politics and has now become one of the most sought-after campaign staffers of the 2016 presidential cycle.

The 40-year-old, who stepped down after nearly four years as executive director of the Republican Governors Association, is credited with playing an instrumental role in expanding the number of GOP governors in a year when they were just supposed to be trying to limit losses. He added to a resume that already included local and statewide victories in Virginia, a key swing state.

Still, Mr. Cox has largely floated under the radar, leaving the political limelight to his boss, New Jersey Gov. Chris Christie.

“The fact that nobody outside of Washington knows his name, other than governors and their staffs, tells you everything about how he approaches his job,” said Mike McKenna, a GOP strategist who has worked with Mr. Cox. “He is not the star; he is the guy behind the scenes doing the work day in and day out. He is very much what he is. He is a blue-collar guy from Massachusetts, and he does the blue-collar work.”

Massachusetts was also the scene of one of his big triumphs last month, when Republicans captured the governorship of the deep-blue state, along with those of Maryland, Illinois and Arkansas, and held onto governorships in 20 of the 22 states they were defending, including surviving tough challenges in Florida, Wisconsin, Michigan, Kansas and Maine.

Phil Cox kept the RGA focused on good policy and good politics,” said Wisconsin Gov. Scott Walker, who was re-elected and could be eying a presidential run. “His leadership was key to our recall win in 2012 and helping a wave of us get elected in 2014. Plus, Phil is just a quality guy.”

Maryland Gov.-elect Larry Hogan, who pulled off perhaps the biggest upset of the campaign season, said that the Cox- Christie duo was “instrumental in helping me achieve victory in a deep-blue state.”

“The fact that we now have 31 Republican governors is a testament to the outstanding job of Phil Cox and his team,” Mr. Hogan said.

Democrats also took note.

“Over the last four years, we’ve gone head-to-head in a number of races,” said Colm O’Comartun, executive director of the Democratic Governors Association. “You win some, you lose some, but Phil was always a true professional, and he did a good job leading his organization.”

Mr. Cox grew up in Hingham, Massachusetts, about 20 miles south of Boston, where his father, Jim, was a pediatrician and his mother, Phyllis, was a social worker. The die-hard sports fan pretended to be part of the Boston Bruins in street hockey games with his brother, and in 1992 his high school hockey team won the Division II State Championship. He played left wing and received the “Mr. Hustle” award.

He caught the political bug not long after he graduated from the University of Virginia when he volunteered for Paul Harris, who was running for the Virginia House of Delegates in 1997.

Mr. Harris said the untested Mr. Cox stood out from all the other candidates. While sitting at a Wendy’s restaurant, he offered Mr. Cox the chance to manage his campaign.

“I looked at him and said, ‘You must be nuts, I have never done this before,’” Mr. Cox recalled. “He said, ‘Neither have I. Let’s do it together.’”

Mr. Harris went on to win the race to become the first black Republican elected to the general assembly since Reconstruction.

Phil’s sense of responsibility was exceptional,” Mr. Harris told The Times. “He was mature beyond his years. He felt personally responsible for his own job performance as campaign manager but also for the performance of the campaign overall.”

Mr. Cox said he found the competition addictive, just like sports.

“There was a winner and a loser,” he said. “All those things were very attractive to a 22-year-old kid out of college. I found I really enjoyed it, and it didn’t feel like work for me.”

Mr. Cox worked on some losing campaigns but also had his share of winners, including managing Bob McDonnell’s 18-point victory in the 2009 Virginia governor’s race.

J. Tucker Martin, a longtime spokesman for McDonnell, said that Mr. Cox “had a clear theory of how that race would be won, and he executed it.”

“You know that when you work with Phil, you know he is going to work harder then anyone you ever met, and if you don’t match it, you won’t be around long,” Mr. Martin said.

He also worked with Tim Phillips, now the head of Americans for Prosperity, met billionaire GOP godfather David Koch and worked for Koch-sponsored groups, including AFP and the Mercatus Center at George Mason University.

In 2010 former Mississippi Gov. Haley Barbour, a major Republican fundraiser, hired Mr. Cox as a consultant for the RGA, where he worked on high-profile gubernatorial races in Florida and Pennsylvania, both of which the GOP won that year.

Before long he was named executive director and has since served under a number of possible 2016 presidential contenders, including Texas Gov. Rick Perry, Louisiana Gov. Bobby Jindal and Mr. Christie.

Despite his success, some wonder whether Mr. Cox will be able to continue his streak of good fortune away from the RGA.

There also are some conservatives who believe he squandered a chance to win the 2013 governor’s election in Virginia by not investing in the race in the final days, leaving GOP nominee Ken Cuccinelli just shy of Democrat Terry McAuliffe.

Mr. Cuccinelli’s camp says Mr. Cox told them he would invest in the race if Mr. Cuccinelli could tighten it. But when the polls closed, Cuccinelli officials say privately that Mr. Cox reneged on the deal and had told donors that putting money into the race would be a bad investment.

For his part, Mr. Cox said, the “ RGA strongly supported the Cuccinelli campaign, investing $8.5 million and accounting for 40 percent of the campaign’s total fundraising.”

“We were disappointed in the result and know Ken would have been a great governor for Virginia,” he said.

Mr. Cox says his focus was on helping his replacement, Paul Bennecke, transition into the job and spending some time at his home in Northern Virginia with his wife, Kim, and his kids, 2-year-old Scarlett and 1-year-old Jordan.

Earlier this month Mr. Cox and Mr. O’Comartun launched 50 State LLC, which was billed as “a bipartisan consulting firm that will focus exclusively on helping private-sector clients achieve results in the states.”

“Whether it be in providing strategic advice, helping with public relations campaigns, or bringing people together to find common ground, we look forward to assisting private-sector clients as they find success in the ‘laboratories of democracy’, the states,” Mr. Cox said in a joint statement.

But that won’t stop him from getting involved in the 2016 race, which he knows is already underway. He said he plans on making a decision about his next move sometime early next year, depending on what sort of offers come his way.

It is clear that he has a soft spot for governors and is particularly fond of Mr. Christie, with whom he has bonded over the past year.

“He is the best communicator we have in our party,” he said. “He can deliver a message and stand in the pocket as well as anyone. He is just as impactful in Jonesboro, Arkansas, as he is Detroit, Michigan. He can campaign in every part of the country.”

Mr. Cox has his eye on running a presidential campaign, and said his time at the RGA allowed him to round out his campaign game.

“I feel like I am at the point in my career where I am a complete player,” he said. “I feel very comfortable delivering sound strategic advice. I know how to raise money. I know crisis communication. I know strategy on the ground, and I am a good manager.

“I feel like if someone asked me to run their campaign for president that I could do it,” he said. “I feel like I am as qualified as anyone else.”