Redistricting has forced two freshmen House Republicans from Arizona to run for the same northwestern Phoenix seat. John McCain, the state’s senior senator, has a favorite. McCain endorsed Ben Quayle, son of the former vice president, for a very McCain-like reason.

In May, Quayle voted for the National Defense Authorization Act, which contained a provision that could lead to the indefinite detention of American citizens charged with but not convicted of terrorism. Quayle’s primary opponent Dave Schweikert voted against the bill. National Journal reported that this simple difference explained McCain’s endorsement.

Quayle has also received the backing of former Secretary of State Condoleezza Rice and raised money with George P. Bush. “It’s kind of like Bush-Quayle Part II—the next generation,” the co-chair of Bush’s political action committee told the San Antonio News-Express.

Not to be outdone, Schweikert has been endorsed by FreedomWorks and various Tea Party activists. The Club for Growth warned GOP leaders in March not to intervene on Quayle’s behalf or they would spend money to help Schweikert. And Schweikert has the support of one group with particularly un-McCain-like foreign policy views: the political action committee associated with Young Americans for Liberty.

Young Americans for Liberty is an organization for constitutional conservatives and libertarians that grew out of Ron Paul’s first Republican presidential campaign. Schweikert told their PAC that in addition to his NDAA vote, he is in favor of troops leaving Iraq, prefers diplomacy with Iran, opposed President Obama’s military intervention in Libya, and believes the Constitution gives Congress the power to declare war.

What interests McCain and YAL isn’t likely to loom very large in next month’s primary. But there is a growing pattern in which economic conservatives and national security hawks find themselves on opposite sides in competitive Republican primaries. Rand Paul versus Trey Grayson was the most obvious example, when Dick Cheney, Rudy Giuliani, and Rick Santorum aligned themselves against a Tea Party candidate largely on foreign-policy grounds.

There have been other examples as well. In Rand Paul’s state of Kentucky, Thomas Massie won a Republican congressional primary with the Club for Growth and FreedomWorks behind him. In 2010, the Club weighed in on behalf of Justin Amash’s successful bid for the GOP nomination in Michigan’s third district.

Neither contest was as high on the neoconservatives’ radar screen as Paul versus Grayson. House members are generally less influential than senators, after all. But suffice it to say that Amash and Massie are not exactly Cheney-Giuliani-McCain Republicans on foreign policy. This is a departure from the not-too-distant past, such as when Iraq war hawks hoped taxpayer groups and anti-earmarks crusaders would help them oust antiwar Congressman Walter Jones in a 2008 GOP primary.

At the moment, this is mostly coincidental in the same way that outfits like the Club for Growth have boosted pro-lifers in Republican primaries. The endorsements are going to the strongest fiscal conservatives. Typically, such candidates are the most socially conservative as well. Recently, some of the sharpest critics of big government at home have been opposing it (albeit more quietly) abroad.

Real conflicts between fiscal hawks and foreign policy hawks may be brewing, however. Sen. Lindsey Graham, McCain’s sidekick from South Carolina, has said he supports a plan that will put tax revenues on the table in order to prevent Pentagon budget cuts. Graham told the New York Times that he had “crossed the Rubicon” on Grover Norquist’s pledge against tax increases.

Norquist for his part has made clear he is willing to see some defense spending sacrificed to avert tax increases. For now, the amount of new revenue Graham is discussing is relatively small and he has emphasized he prefers closing loopholes to raising marginal tax rates. But the sequestration mandated by last year’s debt-ceiling deal and the country’s broader fiscal constraints likely mean this is only the beginning. Cheney is already scheduled to fly from Wyoming to Washington to huddle with the House GOP whip team about the perils of automatic defense cuts.

It is becoming difficult to pay for the neocons’ wars with the supply-siders’ tax rates. That basic mathematical reality could create some infighting within the Republican coalition. The seemingly obscure issues that separate Schweikert and Quayle could become a bigger dividing line within the party.

W. James Antle III is associate editor of The American Spectator and a contributing editor of The American ConservativeFollow him on Twitter.