WASHINGTON — Rep. Vito Fossella’s disclosure yesterday that he fathered a child out of wedlock boosts the chances that the Democratic Party will gain total control of the city’s congressional delegation for the first time in more than a quarter-century.

A week after his arrest for drunken driving, the Staten Island Republican admitted that he had an affair and a 3-year-old daughter with an Air Force lobbyist who retired as a lieutenant colonel.

“My personal failings and imperfections have caused enormous pain to the people I love and I am truly sorry,” Mr. Fossella, who is married with three children, said in a written statement yesterday.

He did not say if he planned to resign his office or whether he would seek re-election this fall. “While I understand that there will be many questions, including those about my political future, making any political decisions right now are furthest from my mind,” he said. “Over the coming weeks and months, I will to continue to do my job and I will work hard to heal the deep wounds I have caused.”

Questions arose about Mr. Fossella’s relationship with the former military officer, Laura Fay, when reports surfaced that he had called her to bail him out of jail after his arrest for driving while intoxicated May 1 in Alexandria, Va. He reportedly told police that he was driving to see his “sick daughter” and take her to the hospital.

The arrest and yesterday’s disclosure could bring a swift end to a once-promising political career for Mr. Fossella, 43, who is serving his fifth full term in the House after first winning an election to replace Susan Molinari in 1997.

While party leaders offered their personal thoughts and prayers to Mr. Fossella and his family yesterday, they withheld political support in anticipation of a decision from the congressman about his future.

Republican officials indicated, however, that a decision must come sooner rather than later.

“I think Mr. Fossella is going to have some decisions to make over the weekend and I would hope and frankly expect that this is a decision between he, his family, and his constituents,” the House Republican leader, Rep. John Boehner, told reporters on Capitol Hill.

Should Mr. Fossella give up his seat before July 1, Governor Paterson would declare a special election for sometime this summer. The winner would have to stand again for election to a full term in November.

The scandal comes at a particularly precarious time for congressional Republicans, who are already preparing for additional losses this fall amid a wave of retirements.

Democrats nationally were already eyeing Mr. Fossella’s seat in New York’s 13th congressional district for a possible pickup before Mr. Fossella’s arrest. If he resigns or decides not to run again, the district would immediately soar up their list of target seats, drawing an infusion of cash from a congressional campaign fund that currently dwarfs that of the Republicans.

“We are ready for an election now or in November,” a spokeswoman for the Democratic Congressional Campaign Committee, Carrie James, said.

For Republicans, possession of the 13th district, which comprises all of Staten Island and slice of Brooklyn, carries larger significance as the party’s only seat in the New York City congressional delegation. “This is a real beachhead in the region for Republicans,” the House editor of the Cook Political Report, David Wasserman, said.

The newsletter yesterday shifted the district from a rating of “likely Republican” to “toss-up,” reflecting Mr. Fossella’s vulnerability.

A Republican loss in Staten Island would mean the city would join Chicago and Boston as major urban centers with one-party representation in Congress, and longtime political observers said yesterday they could not remember the last time Democrats so dominated the five boroughs in Washington. The last Democrat to represent Staten Island in Congress was John Murphy, who retired in 1981.

“It just exemplifies the geographic shift in the base of Republican political power and office holders,” a professor of public affairs at Baruch College, Douglas Muzzio, said, pointing to Republican losses in the Northeast in recent years.

If Democrats gain the seat, New York City would likely look to a veteran Long Island congressman, Rep. Peter King, even more as a go-to lawmaker for funding and other federal initiatives. Mr. King, a former chairman of the House Homeland Security Committee, has often partnered with city representatives to push for more anti-terror dollars from Congress.

“My heart goes out to him and his family,” Mr. King said of Mr. Fossella in an interview yesterday. “He made a terrible mistake.”

Calling Mr. Fossella a “close personal friend,” he cautioned party officials against a rush to judgment and offered a glimmer of hope for his political future. “If Vito wants to run and win again, he can. It’d be a tough fight,” Mr. King said.

Two Democrats are currently declared candidates in the 13th district: a City Council member who has demonstrated fund-raising success, Domenic Recchia, and a Brooklyn attorney, Stephen Harrison, who ran against Mr. Fossella in 2006 and captured 43% of the vote despite being outspent by a margin of 10 to one. With Mr. Fossella damaged, however, other Democrats may jump into the race. Possibilities include a state senator, Diane Savino, and Assemblyman Michael Cusick, both from Staten Island.

If Mr. Fossella resigns or does not seek re-election, Republican hopefuls could include the Staten Island district attorney Daniel Donovan; state Senator Andrew Lanza of Staten Island; and a City Council member of Staten Island, James Oddo.

The local politics are further complicated, however, by the narrow two-seat Republican majority in the state Senate. The effect could be that Democrats and Republicans alike are reluctant to field a state senator as candidate for Congress.