The Republican Party’s already anemic presence in the City Council could shrink after Council Member Dennis Gallagher of Queens, who pleaded guilty yesterday to “forcibly touching and sexually abusing” a 52-year-old woman, resigns his office.

His resignation will take effect at the end of the day on April 18. State election law requires Mayor Bloomberg to call for a special election in the coming weeks to replace Mr. Gallagher, one of just three Republicans in the 51-member City Council.

Local Republicans, trying to avoid losing any seats in the council, are gearing up for a fight. “Oh yeah, I think it’s going to be a race,” the chairman of the Queens County Republican Party, Phil Ragusa, said yesterday. “We don’t want to lose that seat and we’re going to work hard to maintain it.”

Mr. Ragusa said the county party is pinning its hopes on Anthony Como, the commissioner of the Queens County Board of Elections and a former assistant district attorney, to hold the seat. Mr. Como previously ran for state Assembly, losing to Andrew Hevesi, the former state comptroller’s son, in 2005.

A former council minority leader, Thomas Ognibene, who represented Mr. Gallagher’s district for 10 years, has expressed interest in running, City Hall News reported, though he has yet to register as a candidate for the 2009 elections with the city’s Campaign Finance Board.

According to the Queens district attorney’s office, Mr. Gallagher said that on July 8, while he was intoxicated, he “intentionally and forcibly touched intimate parts of the complainant and subjected her to sexual contact without her consent. My conduct was wrong, and I apologize to the complainant. I intend to continue treatment for my alcohol abuse.”

Republicans are saying Mr. Gallagher’s problems should not tarnish the party’s brand as they seek to retain the council seat.

“The sins of the father should not be put on the son,” Mr. Ragusa said.

At least one Democratic challenger has already announced that she will vie for Mr. Gallagher’s seat. Elizabeth Crowley, a program director at the Consortium for Worker Education, a nonprofit labor advocacy group, said she is prepared to run in the coming special election. Ms. Crowley, a cousin of Rep. Joseph Crowley of Queens and the daughter of two former council members, Walter and Mary Crowley, ran unsuccessfully against Mr. Gallagher in 2001, losing by about 5,000 votes. “I feel I was the best candidate then, but it was right after 9/11 and people were concerned, I think, with the fact that I was only 23,” Ms. Crowley said yesterday in an interview. “But I’m 30, and in the past seven years I’ve worked hard in my community. Now my age is on my side.”

According to public filings from January, Ms. Crowley has already raised $70,770 toward a council run, making her a formidable opponent for the Republicans, none of whom have reported gathering any funds yet. Another Democratic candidate registered for the 2009 race, Angelo DiGangi, an attorney, reported that he raised $12,558 in the same period.

Mr. Gallagher is not the first council member to face accusations of sexual impropriety in recent years, although he is the only one to be convicted of a crime. In 2005, Democrat Alan Jennings Jr. of Queens was censured and stripped of his committee posts after five women accused him of sexual harassment and other inappropriate behavior. The previous year, Republican Vincent Gentile was accused of sexual harassment by his chief of staff, but the council took no action.