J. Scott Applewhite/Associated Press

What's a GOP presidential hopeful to do? In the past year, the party has cycled through one favorite aspirant after another before running into a problem: There's no longer a single consensus about what makes a good candidate. Inevitable deviations from conservative orthodoxy are seen as disqualifying sins. Republicans have a habit of killing their darlings.

Ted Cruz seemed to have the right idea. To become the Tea Party's favorite candidate, he outflanked the entire Senate GOP. But that victory came at the cost of a public twice as likely to view him unfavorably as favorably and serious anger from within his own party—so much that it's difficult to envision him winning the nomination in 2016, let alone the presidency. Before him, the immensely popular Marco Rubio was the party's favorite candidate, until he committed the unpardonable sin of working to pass immigration reform. Before that, it was Chris Christie, whom the GOP adored until he got a little too cozy with the president in the wake of Hurricane Sandy. It's enough to make Rand Paul seem like their best option—until you consider that he's angered the Tea Party by supporting immigration reform, the establishment by espousing isolationist foreign-policy views, and his own libertarian base by supporting Mitt Romney in 2012.

Veer right, you're damned; veer left, you're jammed; play it up the center, you're toast.

The cycle of anointment and repudiation echoes the 2012 GOP primaries, when Republicans elevated one candidate after another: Michele Bachmann, Herman Cain, Newt Gingrich, Rick Perry, and Rick Santorum. When voters finally settled on Romney, the candidate had lower favorability and higher unfavorability ratings than any presidential nominee in modern history.

GOP strategist Rick Wilson calls this "the Highlander theory," after the '90s TV show about the Scottish warrior who needs to behead other immortals because there can be only one. Ted Cruz became The One by eclipsing Rubio, who had ascended only a few months earlier. "Ted Cruz and Marco Rubio made similar mistakes in opposite directions," says Ben Domenech, a senior fellow at the Heartland Institute and the publisher of The Federalist. "Rubio obviously tacked toward the center with a push for coming together on immigration policy, and that did damage to his standing with the conservative base. Cruz on the other hand tacked to the right in a way that helped his standing with base but hurt [his] standing with centrists who had been previously open to the idea of him."

This division in the party—with the right driving for purity and the establishment bristling—was most recently evident in the government shutdown. But its imprint is visible in the burgeoning field of Senate and House competitions, too. Almost a dozen Republican House members, such as longtime Idaho Republican Mike Simpson, are facing primaries from the right, with more challenges expected before the cycle begins next year. "This is a key moment for the Tea Party to decide how best to use its resources and whether to really go in behind candidates who need support [against Democrats], as opposed to wasting resources against candidates who have marginal difference from people who might challenge them" from the right, Domenech says.

But the Highlander theory could have the greatest impact on the Senate. Already, six Republican incumbents—Minority Leader Mitch McConnell and Senators Lindsey Graham, Thad Cochran, Michael Enzi, Pat Roberts, and Lamar Alexander—face primary challenges from the right. Most of those seats aren't at risk of a Democratic takeover, but the internecine battles there could distract attention from the task of winning a Senate majority. The real question, Wilson says, is "how much money are you willing to spend to knock off these guys, and how many dollars does [the internal fight] take from [the fight against] Landrieu, Pryor, and Begich? Those guys are getting a free ride because we're more willing to chase purity and keep 40 votes then we're willing to go out and get Democrats that are weak." And there have been rumblings about challenges for other senators, such as Texas conservative John Cornyn, No. 2 in GOP leadership.

"They have to find a way to unify the two sides and leave some neutral ground," Wilson says. "Purity is a lovely thing in soul, but a terrible thing in the real, ugly world of politics that can't be wished away with magical thinking or unicorn dust." There, the only thing that gets wished away is the latest favorite.