Pressure is mounting on Sen. Al Franken to resign, as the number of women accusing the Minnesota lawmaker of groping them grows and fellow Democrats claim the party seems to have a double standard for other members accused of sexual harassment.

Franken, the second-term senator who was accused earlier this month by a Los Angeles radio personality of groping her during a 2006 USO tour, has since been rocked with claims from women who say he grabbed their butts or breasts while posing for photos at events including the Minnesota State Fair.

Earlier this week, yet another woman accused him of touching her breast while posing for a picture. A total of five women have now accused Franken of unwanted touching.

"At the end of the day, I would suspect that Nancy Pelosi is going to have to explain what is the discernable difference between Al Franken and John Conyers." — Arnold Reed, attorney for Rep. John Conyers, D- Mich.

Meanwhile, Rep. John Conyers, the longest-serving member of Congress, has faced multiple claims of past sexual harassment of staffers, including one with whom he reached a financial settlement more than a decade ago.

Leading Democrats, including House Minority Leader Nancy Pelosi, have called for Conyers to step down, prompting his attorney, Arnold Reed, to suggest the party is affording Franken special treatment.

"There are to my count five of these allegations against Al Franken,” Reed said at a Thursday news conference. “There are four — three or four — against the congressman. At the end of the day, I would suspect that Nancy Pelosi is going to have to explain what is the discernable difference between Al Franken and John Conyers. That is a question that she is going to have to answer.”

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For his part, Franken has apologized, but shown no signs of plans to give up his seat. And calls for him to do so have been conspicuously restricted to Republicans.

“I’ve been trying to take responsibility by apologizing,” Franken said at a news conference earlier this week. “And by apologizing to the people I’ve let down. And I’m going to work to regain their trust. I am going to be accountable.”

But late Thursday, the Senate Ethics Committee announced it would begin an investigation into the Franken allegations.

“The Committee is aware of the recent allegations against Senator Al Franken, as well as the calls for an ethics investigation,” the committee said in a statement. “While the Committee does not generally comment on pending matters or matters that may come before it, in this instance, the Committee is publicly confirming that it has opened a preliminary inquiry into Senator Franken’s alleged misconduct.”

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The ethics committee announcement is the first official step on Capitol Hill to hold Franken accountable for his alleged actions. Some Republican senators have urged Franken to at least consider abandoning his seat.

“I think the accusations against him, including many of which he’s admitted, are horrifying,” Sen. Marco Rubio, R-Fla., told CBS Miami last week. “But at this point, he’s going to be before the Ethics Committee. And I would say, in fairness –although the things he’s already admitted to I found to be outrageous and offensive and I do think just on that alone he should consider resigning.”

While most resignation calls have come from Republicans, House Democratic Caucus Chairman Joe Crowley, D-N.Y., said Thursday Franken should step down.

“I think it’s time for Senator Franken to go,” Crowley said in an interview.

The allegations first surfaced in November, when Los Angeles broadcaster LeeAnn Tweeden posted a blog detailing an instance of Franken kissing and groping her in 2006, and shared a photo of what seems to be a grinning Franken standing over her as she sleeps, grabbing at her breasts.

Later that same day, a former Bay Area reporter Melanie Morgan tweeted she had a disturbing experience with Franken, later sharing her account online. Morgan claimed Franken kept calling her and “badgering” her after a dispute they had about budget numbers on a TV show. Unlike Tweeden’s, Morgan’s accusations did not include claims of sexual misconduct.

But next came Lindsay Menz, 33, of Frisco, Texas, who told CNN she was groped by Franken in 2010 at the Minnesota State Fair. Franken, she said, pulled her close when her husband offered to snap a cell phone photo of them, and “as my husband took the picture, he put his hand full-fledged on my rear," Menz said. "It was wrapped tightly around my butt cheek."

Then this week, 41-year-old Army veteran Stephanie Kemplin, of Maineville, Ohio, told CNN Franken groped her in Kuwait during a 2003 USO tour, before the former “Saturday Night Live” performer was a senator. Kemplin said Franken cupped her breast as they posed for a photo.

Two other women have alleged to media outlets that Franken groped them but have requested anonymity. Franken has said he can’t rule out the possibility that more will step forward.

Prior to these allegations, Franken painted himself as a champion of women, speaking out against sexual harassment allegations before Hollywood movie mogul Harvey Weinstein and others.