Speaking of Infidelity …

Democrats can’t be pleased with John Edwards’ confession that he had an affair with Rielle Hunter. It tarnishes the Democratic brand just as they’re gearing up for their quadrennial coming-out party, and the only thing more conspicuous than Edwards’ presence will be his absence. And Edwards could have a Mark Foley-like trickle-down effect in down-ballot races, where negative/positive party associations matter most.

But the news is not all bad for Democrats. First, Obama is pretty much soaked in Teflon when it comes to family matters. Second, it could be a lot worse: What if Edwards had actually won the nomination? And third, it introduces marital infidelity back into the conversation.

Recall: John McCain returned to the United States from Vietnam in March 1973. His wife, Carol, had been in a near-fatal car accident while he was gone. She was overweight, on crutches, and 4 inches shorter than when McCain had left. McCain ended up divorcing Carol for Cindy Hensley, his current wife. Carol has remained mostly silent on her marriage to John, except for one notable comment to a McCain biographer: “John was turning 40 and wanting to be 25 again.”

There were legal complications, too. The Los Angeles Times reported in June that McCain obtained a marriage license while still legally married to his first wife. McCain suggested in his autobiography that he divorced Carol months before marrying Cindy. In fact, that period was about five weeks. He also said that for the first nine months of his relationship with Cindy, he still “cohabited” with Carol. Social conservatives were never McCain’s base, but yes, it could get worse.

For the most part, the media have politely skirted around this episode of McCain’s life. (Not to mention other unflattering moments .) For one thing, it’s long past. McCain has since developed a reputation for credibility and transparency. (Post-Keating Five, that is.) And, unlike Edwards, he told the truth about his deviance. “My marriage’s collapse was attributable to my own selfishness and immaturity,” McCain wrote in his autobiography . “The blame was entirely mine.”

But with Edwards’ infidelity front and center, that could change. In recent weeks, McCain’s ads have taken a turn for the personal, comparing Obama to vapid celebrities like Paris and Britney. Now Obama is coming under increasing pressure to retaliate. The Obama camp has never publicly raised McCain’s marital issues, nor would it. But insinuation, coupled with euphemisms about “trust” and “commitment,” can go a long way. In an environment filled with personal attacks on both sides, you can bet McCain’s past will become fair game. The Edwards news even gives McCain’s detractors a convenient pretext to raise the subject. So you heard about Edwards ditching his sick wife? Wait till you get a load of McCain …