Yesterday, Sen. John Ensign, R-Nev., the fourth-ranking member of the Senate leadership, admitted that he had an affair with a staffer, whose husband also worked for him.

Ensign, an evangelical Christian and vocal moralizer against gay marriage, was said to be planning to run for president in 2012. He will also be up for reelection to the Senate that year.

Ensign was separated from his wife in December 2007 when he began the affair, reportedly with Cynthia Hampton, a campaign staffer whose husband Dough worked on Ensign’s Senate staff:

[Cynthia Hampton] served as the treasurer for Ensign’s reelection campaign and for his leadership fund, Battle Born PAC. According to people familiar with the matter, Ensign’s affair with Hampton took place between December 2007 and August 2008. FEC records show that she ended her affiliation with the two committees in early 2008.

Hampton is married to Douglas Hampton, who, according to Senate records, served as Ensign’s administrative assistant in his personal office from November 2006 to May 2008 — around the same time Cynthia Hampton left Ensign’s committees.

Politico also reports that Ensign may have come forward about his adultery because the Hamptons were trying to extort money from him.

Citing an anonymous source, the AP reports that Ensign also had an extramarital affair in 2002.

Ensign has been a vocal opponent of gay marriage, who typically couches his opposition in terms of moral sanctitude. For example, in July 2004, his office sent out a news release under the headline, “Ensign Defends Sanctity of Marriage on Senate Floor,” that included a quote from Ensign in the floor debate:

Marriage is the cornerstone on which our society was founded…Marriage, as a social institution, predates every other institution on which ordered society in America has relied… It is not right to mold marriage to fit the desires of a few, against the wishes of so many, and to ignore the important role of marriage.

In June 2006, Ensign voted for the Federal Marriage Amendment, which would have established gays as a class restricted from the right to marry, primarily based on the assertion that homosexuality is a sin.

Eighteen months later, he began an affair with a married woman.

As sins go, adultery, which is prohibited twice in the Ten Commandments (overtly in number 8, “You shall not commit adultery,” and by inference in number 10, “You shall not covet your neighbor’s wife”) is a much more serious sin than being gay, which is referenced in a few obscure passages in category of “abominations” that also includes eating ham, shrimp and lobster and women wearing pants.

Ensign has made a practice of speaking out on the adulteries of other politicians:

In 2007, he called on Sen. Larry Craig, R-Idaho, to resign after Craig was charged with soliciting sex in an airport men’s room.

And yet later that year, when Sen. David Vitter, R-La., admitted that he’d consorted with prostitutes, Ensign — who was chair of the Republican Senate Campaign Committee — was criticized for saying Vitter should not resign:

[Ensign] said Craig “admitted guilt. That is a big difference between being accused of something and actually admitting guilt.”

“David Vitter never did that. Larry Craig did,” continued Ensign on ABC’s “This Week” program.

Sen. Patrick Leahy, D-Vt. and chairman of the Senate Judiciary Committee, expressed a contrary view on “Fox News Sunday.”

“One, I say there’s a double standard,” said Leahy. “Secondly, I don’t think they’ll ask him (Vitter) to resign because, of course, he’d be replaced by a Democrat. It’s easier to ask Larry Craig to resign because he’d be replaced by a Republican.”

Idaho has a Republican governor who will appoint a successor to Craig. Louisiana’s governor is a Democrat.

In the 1990s, Ensign called for Pres. Clinton to resign after his extramarital affair came to light:

“I came to that conclusion recently, and frankly it’s because of what he put his whole Cabinet through and what he has put the country through,” he was quoted saying at the time. “He has no credibility left,” he added.

Despite his rank hypocrisy on this issue, it is unlikely there will be calls for Ensign to resign — or that the confession will even hurt his career. While admissions of gay affairs tend to ruin anti-gay Republicans — Rev. Ted Haggard and Sen. Craig, for example — heterosexuals like former House Speaker Newt Gingrich, former New York Mayor Rudolph Giuliani, former House Majority Leader Tom Delay often survive and even thrive. For example, Sen. Vitter is said to have a better than even chance of being reelected next year.