Jay Rosen has written a thoughtful analysis regarding the apparent invulnerability of the Trump campaign to the traditional gauntlet of media fact-checking. Rosen argues—in response to other journalists quoted at the beginning of the piece—that the press has not necessarily become less powerful than before; instead, he contends, the existing limitations of the media's ability to shame politicians and campaign consultants into remaining (relatively) truthful have been exposed by a candidate who uniquely violates the standard norms of campaign conduct. Attempts by journalists to referee Trump's claims have, says Rosen, simply failed to breach a brazen indifference to reality that is without precedent in modern politics. Because Republican voters are already unlikely to trust the "mainstream media," efforts to fact-check Trump simply seem to validate his accusation that he is the victim of hostile journalists.
This seems fairly convincing, but I think there's another angle here that's important. It's difficult for journalists to successfully call politicians on their incorrect or misleading claims in the absence of political opponents who are doing the same. If prominent members of the opposite party (or, in a primary election, rivals within the same party) are not either leading or echoing any public charges of insufficient veracity, such accusations are unlikely to stick—especially because journalists soon become uncomfortable with banging on about an issue that even a politician's natural enemies don't seem to treat as damning. (The standard manner of structuring these news stories is the form "X's Behavior Raises Questions," which requires the authors to specify in whose minds these questions are being raised—preferably, not just their own.) On the Republican side, some conservative media figures may enjoy sufficient credibility with their audience to inflict damage on a candidate in the absence of criticism from fellow politicians, but it is unlikely that the "mainstream" press has such influence within the GOP.
To me, what makes Trump different is not that he is less truthful than other candidates but that he has attracted remarkably little direct criticism from other Republicans despite his consistent lead in the polls (with the exception of Carly Fiorina and John Kasich, but neither is prominent or popular enough to generate much attention at this stage). In part, this is because some of his statements would be politically inconvenient for other Republicans to dispute without calling their own conservative credentials into question (e.g. his negative characterizations of immigrants and Muslims). His rivals also appear to view his candidacy as likely to fall of its own weight, and they want to be in position to corral his supporters for themselves when it does—which would presumably be harder to do if they antagonize him now. Though Trump's behavior reliably scandalizes commentators and activists on the left, even elected Democrats have mostly held their fire so far—after all, it's not in their party's interest to puncture the Trump bubble before he actually becomes the Republican nominee.
In the competitive world of politics, it is rare for a candidate to rise as far as Trump has in the polls without withstanding a sustained line of criticism from other politicians. But here we are. Trump may be an unusual case, but so is a strategic environment in which few trailing candidates perceive an incentive to aim sharp attacks at the leading contender. If he is still ahead in January, however, desperation will set in among the rest of the Republican presidential field (to say nothing of the vast majority of the national party leadership). At that point, voters may start to hear a lot more about his apparent problems with the truth.