"He's not afraid to make a choice. He sees a candidate who he thinks will be good and have a shot at winning, he's going to get involved," said Tom Rath, a longtime Romney adviser who remains in touch with the former presidential nominee. "He's a very smart guy. He understands the process now, and sees the difference outside validation can make if it's done the right away and it's done early enough."

To be sure, Romney's effectiveness stems more from the GOP establishment's growing success over grassroots conservatives, more than his own individual influence. His favored candidates include many longtime allies, including Virginia Senate candidate Ed Gillespie, Romney's 2012 senior adviser; Virginia state Delegate Barbara Comstock, a Romney staffer in 2008; California state Senator Tony Strickland, a former Romney California state chairman running for Congress; and Stefanik. But they also contain some up-and-coming prospects, like Ernst and Wehby, along with leading Senate challengers like former Senator Scott Brown in New Hampshire, Representative Bill Cassidy in Louisiana, and Representative Steve Daines in Montana.

Romney's successful post-presidential campaign involvement is a reminder that for all the Tea Party influence within the GOP, there's a "Romney wing" of the party that's alive and well even past the candidate's sell-by date. The Washington Post's Dan Balz wrote a thought-provoking column in March, arguing that "somewhat conservative" voters make up the largest faction within the party. "These voters like candidates with governing experience who have conservative values but do not push radical policies and are optimistic about the country. They reject culture warriors," Balz wrote. It's a good description of the candidates Romney has backed this election year.

Indeed, the lineup of Republican Senate candidates in 2014 reflects a degree of country-club Republicanism, based on their biographies, if not their ideology. Three of the party's nominees have, like Romney, career experience with blue-chip consulting firms. In North Carolina, Thom Tillis served as a partner at PriceWaterhouseCoopers before pursuing a political career, first running for office to get a mountain-bike path built in his hometown. Conservative favorites Ben Sasse and Tom Cotton spent stints at McKinsey & Company, employing Romney-like career tracks before jumping into politics.

Romney himself will be attending a National Republican Senatorial Committee fundraiser on June 9 in New York, according to the Post. Later in the month, he's hosting a retreat on the future of American leadership at his mansion in Park City, Utah, inviting leading prospective contenders in 2016 to attend.

"Mitt's the closest thing we have to a party elder right now, someone who's been through the fire. His stature is presidential, and I think people miss that," said Rath. "He lost an election, but he didn't lose interest. He really does have concern about the direction the country's headed."

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Josh Kraushaar is the politics editor for National Journal. Connect Twitter