With that said, journalists could be more effective in responding to a pattern of false claims. First, they should remember to continue to remind readers—some of whom are just starting to tune into the campaign—that claims like those in the welfare ad are bogus. Jeff Zeleny and Jim Rutenberg did exactly that in a New York Times report last weekend that flatly described the ad as “falsely charging that Mr. Obama has ‘quietly announced’ plans to eliminate work and job training requirements for welfare beneficiaries.” Likewise, in a blog item that was later published in print, the Times’s Michael Cooper reminded readers that the Republican convention featured a “selectively edited” clip of President Obama’s “you didn’t build that” statement, which was made all the way back in July.

Second, as The Atlantic’s Garance Franke-Ruta has argued, reporters should cover a pattern of false claims as an ongoing story rather than ignoring it as old news. For instance, a widely lauded Los Angeles Times story highlighted by Rosen focused on former Pennsylvania senator Rick Santorum’s repetition of the false welfare claim in his speech at the convention Tuesday. Under the headline “Rick Santorum repeats inaccurate welfare attack on Obama,” the LAT’s David Lauter notes in his third paragraph that Santorum’s “reprise of an inaccurate Romney campaign attack on Obama over welfare” gave his speech “its hardest edge.” This is the sort of story that, over time, can threaten a politician’s reputation for truth-telling.

But while journalists have recently produced some strong work about truth and lies in the campaign, the morning-after coverage of Ryan’s speech in the mainstream media is largely an example of what not to do. Leading outlets largely buried or ignored the vice presidential nominee’s hypocritical and misleading criticism of Medicare spending cuts (which his own budget assumes) and his attack on Obama for not embracing the recommendations of the Simpson-Bowles deficit commission (which Ryan himself opposed). Instead, the reporting tended to focus, as The Daily Beast’s Howard Kurtz did, on theater critic-style analysis of the effectiveness of the speech. (Kurtz slipped a brief discussion of Ryan’s “cynically selective attack” on Obama into the eighth paragraph of his analysis.)

Of course, there is no magic bullet here. Given current levels of polarization and media distrust, many voters will remain unpersuaded by factchecks, which in turn reduces the incentive for politicians to care what the media says. But journalists rightly espouse a creed that their highest duty is to the truth, not the marketplace or the people they cover. When someone who could be the next president or vice president of the United States makes a false claim, it is always a newsworthy act. Reporters should honor that duty in their coverage.

Brendan Nyhan is an assistant professor of government at Dartmouth College. He blogs at brendan-nyhan.com and tweets @BrendanNyhan.