David Vitter
Apparently Senate Republicans draw a sharp line between adultery with a staffer and adultery with a prostitute: The former leads to resignation from leadership positions, the latter not so much.
Or so the examples of John Ensign and David Vitter would seem to suggest. From the Louisiana Democratic Party:
The Louisiana Democratic Party is today calling on Sen. David Vitter to resign his leadership position in the Republican Party after Sen. John Ensign stepped down from the chairmanship of the Republican Policy Committee, the fourth ranking spot in Senate leadership. Sen. Ensign resigned after confessing to an extramarital affair with a campaign staffer. Sen. Vitter confessed to a "very serious sin" involving illegal activity after his name was mentioned in the federal investigation of the D.C. Madam prostitution ring, and the Louisiana Democratic Party is calling on him to resign his Deputy Whip position in the Senate.
No dice there, though; Vitter's staying put. The most obvious interpretation is therefore that what Ensign did was worse (though Vitter's was still a very serious sin!). But a Louisiana pollster quoted in Roll Call has another theory:
“I don’t think this will help or hurt Vitter,” Pinsonat said. “If anything, it leans towards helping him because ... the more this stuff happens the more it becomes ho-hum. You can’t say it’s just David Vitter. ... It happens so often, I don’t think it’s as stunning an event as it was 15 years ago.”
So two lessons to keep in mind when planning your adultery: Better a professional than an employee, and if you're lucky enough to be a Republican lawmaker, thanks to the efforts of Vitter, Larry Craig, Newt Gingrich, John Ensign, and so many others, you are now good to go.