SHINING A LIGHT ON A ‘SERIOUS SIN’…. When it comes to Sen. David Vitter (R), seeking re-election in Louisiana this year, the question was never whether his humiliating scandals would be a campaign issue. The question was how Democrats would make the case to voters that Vitter’s dishonesty and character problems are a key campaign issue.

Yesterday, we got a pretty clear sense of the pitch. Vitter’s Democratic opponent, Rep. Charlie Melancon, unveiled one of the season’s hardest-hitting ads, shining a light on Vitter’s background with prostitutes and hiring an abusive criminal to oversee women’s issues for his Senate office. Just as importantly, Melancon’s ad ties these scandals to Vitter’s votes against equal pay for women workers, against mandatory coverage for mammograms, and against protections for women who are raped on the job.

The closing line is pretty devastating: “David Vitter: for women, his ‘serious sin’ isn’t even his worst.’” It coincides with the Melancon campaign launching a new website:

The ad serves multiple purposes. The obvious goal is to remind voters of Vitter’s character problems and poor judgment, especially when it comes to women. Even if voters aren’t moved by Vitter running on a family-values platform and then getting caught with prostitutes, this ad connects that scandal to a larger pattern that makes the right-wing senator untrustworthy.

What’s more, as Jed Lewison noted, Melancon’s larger goal “is to make this election a referendum on Vitter. If he can do that, he actually stands a shot at winning.”

I’d add that it’s very likely Melancon’s only shot at winning. Given Louisiana’s political direction in recent years, and the general tilt of the electorate in 2010, Vitter’s far-right record and hostility to the middle class probably won’t be enough to undermine his re-election bid. The key is to make the case that the senator simply isn’t trustworthy — a case that includes his background cheating on his wife with hookers, but as the ad shows, goes further. If this doesn’t undermine Vitter’s support, it’s very likely nothing will.

In an item yesterday, Chris Cillizza compares this race to Blair Hull’s (D) Illinois Senate race in 2002 and Nikki Haley’s (R) South Carolina gubernatorial race this year. Neither of these comparisons work for me — Hull’s controversy had to do with a messy divorce, and allegations of Haley’s adultery weren’t proven, and certainly didn’t involve prostitutes.

The more apt comparison may be to former Sen. Tim Hutchinson (R) in Arkansas, who ran for re-election in a Republican state during a very good year for Republicans, but lost after getting caught up in a sex scandal, despite running on a family-values platform.

Granted, Sen. Mark Pryor (D) was running for a seat once held by his father, and Melancon doesn’t have those kinds of advantages. But the point is, it’s possible.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.