House Republicans might not like the idea of one of their leaders picking favorites in Member-vs.-Member primaries, but at least for Majority Leader Eric Cantor, his backing of a winner in the Illinois GOP primary decision has only solidified his power base.

In the wake of Rep. Adam Kinzinger’s victory over Rep. Don Manzullo on Tuesday — winning 56 percent to 44 percent — Congressional observers note that despite some short-term gripes directed at Cantor, in the long run, his effort in Illinois could pay off for his own ambitions. It also poses a threat to outside groups who jealously guard their own status as kingmakers.

“It’s about, ‘Alright, I want to be a successful Speaker, and I’m going to lay it down now over a long period of years and be in a very strong position,’” one GOP lawmaker said.

The lawmaker also said Cantor’s move further indicates “he’s playing with real bullets and he has the capacity to affect outcomes. Is that such a bad thing?”

Cantor has made recruiting young, conservative candidates a top priority and helped bring dozens of them to Congress during the 2010 cycle.

“Eric is the only member of leadership to endorse a presidential candidate that I’m aware of, and so I wasn’t shocked,” Rep. John Campbell (R-Calif.) said. “Not because of who was in the race, but because Eric tends to make his opinions known. He will err on that side rather than keeping it quiet.”

The YG Action Fund super PAC — run by a former Cantor aide — spent $52,000 on a radio ad boosting Kinzinger, the freshman who ran against the 10-term Manzullo and whom Cantor personally endorsed.

Cantor’s move did not go unnoticed, particularly among conservatives who have caused their own stir in GOP primaries. On Wednesday, Club for Growth President Chris Chocola warned House leaders not to show favoritism in the Arizona primary between GOP Reps. David Schweikert and Ben Quayle. In a letter, the former Indiana Congressman cited the Illinois primary and called for neutrality in Arizona.

“Should it become apparent that you are choosing sides on behalf of Rep. Quayle, the Club for Growth PAC will consider it necessary to intervene on behalf of Rep. Schweikert,” Chocola wrote. “As is our practice, if the Club’s PAC entered this primary, it is highly likely that our 75,000 members would donate considerably more funds to Rep. Schweikert’s campaign than the Republican House leadership would contribute to Rep. Quayle’s campaign.”

The club letter was the most visible sign of unrest Wednesday, in addition to grumblings from some rank-and-file Members. Other Republicans said Manzullo’s demands for Cantor’s resignation and claims of widespread unrest over his endorsement produced headlines but predicted no big drop-off for Cantor.

Kinzinger’s decisive victory, winning by 12 points when many observers predicted a far closer race, could also help soothe any hurt feelings among senior Members who were wary of a veteran lawmaker being targeted for defeat. The rationale goes that given how handily Kinzinger defeated Manzullo, those Members might simply see it as Cantor recognizing the reality that “it was time for Manzullo to retire,” one leadership aide said.

Some veteran lawmakers were left wondering Wednesday “if they may be next,” another GOP Member said.

The next likely GOP Member-vs.-Member primary is Aug. 14 in Florida, where another freshman lawmaker, Rep. Sandy Adams, is likely to face veteran Rep. John Mica. The Quayle-Schweikert race is Aug. 28. And Reps. Charles Boustany and Jeff Landry face off in Louisiana’s Nov. 6 nonpartisan primary.

Furthermore, although the issue of the Illinois primary was not broached in the Conference meeting this week, according to a source at the meeting, some predict it will surely be raised next week when Manzullo returns. How things play out from there will be more telling, sources said.

“This will undoubtedly cause some consternation,” a GOP strategist said. “From the rank-and-file Member’s perspective, leadership is not elected to be in the business of picking winning and losers within the Conference. From a leadership perspective, it also requires spending a lot of political capital, perhaps to an undesired effect.”

Adding another wrinkle is whether a new group, the Campaign for Primary Accountability, will continue to make its presence felt in these primaries. The super PAC spent a whopping $222,000 on Kinzinger’s behalf late in the race. The six-figure sum is more cash than the controversial group has spent in any other race this cycle.

The super PAC spent at least $75,000 on television ads and $18,000 on radio spots knocking Manzullo and pumping up Kinzinger, according to Federal Election Commission records. Online reports show it also unloaded at least $104,700 in direct mail on the race through postcards and other pieces.

CPA targets incumbents in primaries in safe districts, regardless of party affiliation, according to spokesman Curtis Ellis. Its most notable success was spending against Rep. Jean Schmidt (R-Ohio), who lost her primary earlier this month.

Abby Livingston and Shira Toeplitz contributed to this report.