No one, including Reid, has seriously contested the claim that his accusation is unsubstantiated. Instead, Romney opponents and critics of factchecking organizations have relied on a series of semantic maneuvers to try to change the issue. This approach has often been successful—in part, it seems, because the GOP nominee has breached a norm in not releasing more tax returns, creating an impression among reporters that he has something to hide.

Place the burden of proof on Romney

Some Romney critics have suggested that the former Massachusetts governor’s failure to release additional tax returns somehow legitimizes unsubstantiated claims about their contents. A related approach is to claim that Romney is obligated to disprove the allegations since he failed to release previous tax returns. But as I wrote on Twitter, I can’t imagine that Reid defenders would rise to defend a public figure who was making claims about, say, President Obama’s college transcripts based on an anonymous tip (see, e.g., Donald Trump’s tweets yesterday). While Obama’s transcripts are not in serious question beyond the fringe, the guilty-until-proven-innocent logic being applied to Romney would suggest that Obama’s very refusal to release them implicates him. After all, Obama could easily disprove any such claim by releasing his transcripts! Consider another example: if John McCain had not released his medical records, would it have been acceptable for Democrats to accuse him of dementia or a recurrence of melanoma based on an anonymous tip? Again, McCain could have disproved such a claim. By the logic of Reid’s defenders, virtually any accusation that McCain could have refuted would therefore be justified.

Attack truth ratings and exploit the difficulty of proving a negative

The Pulitzer Prize-winning factchecking website PolitiFact looked into Reid’s claim and found “no evidence to back [it] up.” Indeed, they quote three experts calling it “preposterous,” “extremely unlikely,” and “highly unlikely.” Rather than focus on these findings, however, critics are exploiting PolitiFact’s flawed rating system, an increasingly common tactic among targets of the group’s critiques and their defenders. In this case, the site rated Reid’s claim “Pants on Fire,” prompting a frustrating chorus of protests on blogs and Twitter that PolitiFact can’t prove Reid’s statement is false. That’s of course true, but misses the point.

Imagine that Mitt Romney said an anonymous source told him that Iraq’s weapons of mass destruction had been moved to Syria before the US invasion. If PolitiFact looked into that claim, they would find no credible evidence to support it, and experts would tell them that the prospect of hidden Iraqi WMD is, well, “preposterous” and “extremely unlikely.”

The problem is that PolitiFact’s rating system doesn’t work well for irresponsible and unsubstantiated claims that can’t be definitively falsified (and likewise Glenn Kessler’s four Pinocchio rating of Reid’s statement at The Washington Post). The site is right to hold public figures like Reid accountable for making such claims, but the standard to which they are held does not easily map onto a scale of truth and falsehood. For instance, it is impossible to prove that a “Pants on Fire” rating for Reid is merited. As a result, critics can divert attention from the substance of PolitiFact’s analyses and turn the debate into a referendum on the epistemological flaws in the site’s ratings, which force complex issues into arbitrary and subjectively determined categories. MSNBC host Rachel Maddow, for instance, acknowledged that Reid’s claim is “totally unsubstantiated,” but still used the PolitiFact rating as a vehicle to dismiss the site, telling her viewers that “it’s ok to not take them seriously on anything ever.” A better approach would consider whether claims can be supported and whether they are consistent with the best available evidence without assigning labels to them (full disclosure: this is the approach we used at Spinsanity).

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