A leading national gay rights advocacy organization is pressuring Sen. Barbara Boxer (D-Calif.), chairwoman of the Senate Ethics Committee, to drop an investigation into allegations of sexual misconduct by Sen. Larry Craig (R-Idaho). As a result, Democrats may question the merits of pushing the embattled Republican out of Congress.

The National Gay and Lesbian Task Force, which last week helped push gay rights legislation through the House, has written a strongly worded letter to Boxer and Sen. John Cornyn (Texas), the Republican vice chairman of the ethics panel, criticizing their investigation of Craig as unfair.

The outspoken critiques by national gay rights leaders puts pressure on Boxer to reconsider an ethics committee action that gay political leaders view as a witch hunt motivated by the homophobia of GOP Senate leaders.

Boxer may be more responsive to their criticisms than many of her colleagues because she represents San Francisco, home to one of the largest gay communities in the nation. Her home state is also considered one of the most politically progressive in the nation.

“We are writing to state the inherent contradiction between your treatment of allegations of ethical misconduct by Senator Larry Craig and Senator David Vitter and to insist that you open an investigation into Sen. Vitter’s conduct,” wrote Matt Foreman, vice chairman of the National Gay and Lesbian Task Force.

“There is no explanation for the diametrically opposed responses to these two situations, other than hypocrisy tinged by homophobia,” the letter said. “There are only two ways to resolve this: drop the investigation into Sen. Craig or investigate the allegations surrounding Sen. Vitter.”

The letter will be delivered to the senators Tuesday, said Foreman.

Vitter aides did not respond to e-mailed requests for comment.

The Select Committee on Ethics quickly launched an investigation of Craig once it came to light in late August that he had pleaded guilty to disorderly conduct after falling prey to a gay sex sting in a Minneapolis airport bathroom. An undercover police officer accused Craig of swiping his hand and sliding his foot beyond a bathroom stall divider in a ritual of coded solicitation.

Craig said he was merely picking up a piece of paper from the ground and naturally has a “wide stance.” Craig said he pleaded guilty to avoid public exposure.

The scandal threatened to damage the GOP’s image as the party of traditional moral values. GOP leaders pressured him to resign and asked the ethics committee to investigate his arrest.

Craig’s Senate spokesman referred a request for comment to the lawmaker’s attorney, who did not respond to two phone messages.

Boxer and Cornyn later informed GOP leaders in a publicized letter that they would investigate Craig as long as he continued to serve in the chamber.

But there has been no evidence the committee has taken up a probe of Vitter, even though the Louisiana lawmaker’s phone number was found in the records of Deborah Jeane Palfrey, the infamous D.C. Madam, whom federal prosecutors have accused of running a prostitution ring in the nation’s capital. Vitter apologized for his association with Palfrey’s escort service but has refused to provide details.

Since then, allegations have surfaced that Vitter consorted with prostitutes in New Orleans. Jeanette Maier, who was convicted in 2002 for running a brothel, has claimed Vitter was a client.

Vitter has denied these claims.

There is no evidence that GOP leaders or the ethics committee has taken any action against Vitter. Instead, his colleagues gave him a standing ovation after he apologized during a luncheon meeting of the Senate Republican Conference.

Gay rights leaders say the disparate treatment is evidence of a double standard.

“The fact that Sen. Vitter has not yet been charged in a criminal court is not relevant,” wrote Foreman of the National Gay and Lesbian Task Force in his letter to Boxer.

Foreman noted Vitter’s link to the D.C. Madam and the allegations of Louisiana prostitutes.

“Your committee has the authority — indeed, the obligation — to investigate these serious matters to determine what course of action is appropriate.”

Chuck Wolfe, president of the Gay and Lesbian Victory Fund, the nation’s largest gay and lesbian political action committee, said that anyone with a personal sense of ethics would see the probe as unfair.

“Certainly there’s a double standard because everyone’s assumption is they are dealing with one [lawmaker’s transgression] because it’s homosexual sex and not the other’s because it’s heterosexual sex,” said Wolfe, in reference to the ethics committee. “If they investigate every member of the U.S. Senate because of an extramarital affair, gay or straight, we’d be getting even less done than we are today.”

“Anyone with any degree of personal ethics will drop it,” said Wolfe.

The ethics committee has not appeared to move against Vitter despite a complaint filed in July by Citizens for Responsibility and Ethics in Washington (CREW), a left-leaning ethics watchdog group.

Naomi Seligman Steiner, CREW’s spokeswoman, said the ethics panel has shown a double standard.

“It’s outrageous that they don’t seem to be investigating,” she said. “They seemed to jump so quickly with Mr. Craig and [lawmakers] gave Sen. Vitter a standing ovation in the Republican caucus.”

“What is shocking is that [Ethics] moved so quickly and so publicly with Craig, so it clearly is a double standard,” she said. “The problem with the ethics committee not starting an investigation is that we don’t know all the facts.

We’re not really getting to the bottom of it and why Vitter’s being protected.”

Boxer’s spokeswoman declined to comment.