In last night's debate, John McCain claimed that ACORN "is now on the verge of maybe perpetrating one of the greatest frauds in voter history in this country, maybe destroying the fabric of democracy."

That's quite an allegation against a group that's working to register low-income voters. You'd hope that the media would ask McCain's campaign for some evidence for the claim, or at least note that the candidate himself didn't offer any. Or that moderator Bob Schieffer would have followed up in real time.

You'd be disappointed, of course. Reporters were too distracted by Joe the Plumber to pay much attention to McCain's hyperbolic accusation.

Of course, McCain had essentially no backing whatsoever for his claim. As TPMmuckraker and others have pointed out, there's virtually no evidence that fraudulent registration forms of the type erroneously submitted by ACORN in their thousands in some states ever turn into fraudulent votes.

(Indeed, the whole voter fraud controversy is such baloney that now even Florida's Republican governor Charlie Crist, a big McCain backer in the primaries, felt compelled to throw some water on it, telling reporters yesterday: "I think that there's probably less [fraud] than is being discussed. As we're coming into the closing days of any campaign, there are some who enjoy chaos.'')

But the media's failure last night is in keeping with its broader failure to explain that key distinction between voter registration fraud and voter fraud point. We've highlighted some egregious examples of CNN conflating the two. But there are plenty more from other outlets.

Here's a report from ABC's World News Tonight, flagged by Media Matters, which aired Tuesday night, in which correspondent Jake Tapper, keying off claims made by McCain, sounds the alarm about "voter fraud." Rather than stating authoritatively that the fraudulent forms aren't going to lead to fraudulent votes cast, the story goes he-said she-said, leaving it to Barack Obama to say it while talking to reporters about the charges -- as if this were a debatable point, when in fact it's a crucial fact which undermines the essential premise of the story.

Or consider this NBC News "Deep Background" investigative report, which stokes fears of voter fraud by running down ACORN's history of legal disputes over its registration activities, without ever explaining that in not a single one of these cases was there evidence that fraudulent voting took place.

It's thanks largely to this ongoing media failure that the McCain camp is continuing to flog the issue. Already today, Sarah Palin told a crowd in Bangor, Maine that voters face "a choice between a candidate who won't disavow a group committing voter fraud and a leader who will not tolerate the voter fraud."

Given how clueless the reporting on this story has been, it's almost hard to blame them.