Garance Franke-Ruta has three suggestions for how the media can handle repeated campaign lies (such as Mitt Romney's claim that Obama has eliminated the work requirements from welfare).
1) Add boilerplate to every story indicating it is a lie (similar to how stories about "Obama is a Muslim" always included language informing the reader that this was false).
2) Always attribute the charge to campaign partisans, and immediately quote rebuttal from opposing partisans.
3) Turn repeated lying into its own story.
Of these, I dislike the second, because it's identical to he said/she said journalism, which actually allows the lie to get traction because independents will assume the truth lies somewhere in the middle. But the other two proposals are spot on. Franke-Ruta expects us to start seeing "repeated liar" stories, which would surprise me. It's hardly unprecedented -- remember Election 2000? -- but it seems like it takes balls the media tends to lack.
I'd be particularly intrigued if such a story didn't just attack the credibility of the instigating campaign, but also took on the think tanks providing a "source" for the charge (in the case of the "Obama gutted work requirements" lie, that would be the Heritage Foundation). Such organizations depend, in large part, on their ability to be taken as credible currency by the media -- providing an "independent" veneer to whatever claim they're making. I'd love to see some consequences when they abuse that privilege and act as simple hacks.