Don't look now, but John McCain is in trouble.

The latest Rasmussen poll finds that 52% of Arizona Republicans support Sen. McCain (R-AZ), while 40% prefer former Republican Congressman J.D. Hayworth, with a 4.5% margin of error. While a twelve point lead might seem commanding, incumbents with just 50% support—which McCain has been hovering around—are usually considered vulnerable. McCain is still likely to pick up enough undecided voters to win Arizona's August 24 primary. As Nate Silver argues, the so-called "incumbent rule" that undecided voters tend to break for the challenger really only applies to last minute polling. But because incumbents are a known quantity in a way their challengers usually aren't, McCain isn't likely to pick up that much new support before the primary. And Hayworth has three months to make up the difference.

It can't be lost on McCain that a 30-year veteran of the Senate, Arlen Specter (D-PA), just lost a primary challenge of his own. Like McCain, Specter led in most of the early polling, but was unable to get much above 50% support. As Chris Cilizza reports, even McCain allies admit that the immigration issue and dissatisfaction with incumbents make this the most serious challenge to McCain's chances of holding onto his seat since he first ran for Congress in 1982. Specter's case is different, of course, because unlike McCain Specter actually switched parties last year, in the process alienating Republicans without ever really endearing himself to Democrats. But just as Specter had a hard time reinventing himself as a Democrat, McCain is having a hard time reinventing himself as a movement conservative.

Now McCain, who at one point sponsored a bill that would have granted illegal immigrants "amnesty"—now a dirty word among Republicans—supports Arizona's controversial new immigration law and demanded the government "complete the danged fence" along our borders after being attacked by Hayworth for being soft on immigration. And after calling himself a "maverick" for years—his 2003 memoir is subtitled "The Education of a Maverick"—McCain recently told Newsweek that he never actually considered himself a maverick. It's a claim that Jon Stewart describes as equivalent of "I Can't Believe It's Not Butter saying, 'I never believed I was butter.'" The truth, of course, is that just as McCain's only chance of winning the presidency in a year when voters viewed Republicans unfavorably was to run as an atypical Republican, his only chance of getting back to the Senate now depends on him proving that he is a mainstream Republican after all.

Say what you will about McCain, but he's a survivor. He still has millions more dollars in the bank than Hayworth does. And whoever survives the Republican primary is still likely to win the general election in the fall. McCain may certainly pull out a victory yet. But it is striking that last year's Republican nominee for president is struggling to hold onto his Senate seat against a challenger from his own party. After suffering a resounding defeat in the last presidential election, the Republican Party has moved back to the right. And McCain is starting to seem like the party's past, not its future.