WASHINGTON — When Mayor Bloomberg testifies here this morning before a House subcommittee, lawmakers will hear a familiar plea: The city needs more federal aid for rescue workers who toiled at ground zero after the terrorist attacks of September 11, 2001.

The difference today will be a new voice added to the clarion call: that of Rep. Peter King, the veteran Long Island Republican familiar to many New Yorkers for his hard-line stance on homeland security and illegal immigration.

Mr. King will appear alongside Mr. Bloomberg and Reps. Jerrold Nadler and Carolyn Maloney as he joins the push for funding to monitor and treat illnesses linked to the aftermath of the terrorist attacks on the World Trade Center.

The testimony on the September 11 health issue is a first for the eight-term lawmaker, but it probably will not be the last. With the likelihood growing that the city will lose its lone Republican representative in Congress, Mr. King, 64, has quietly emerged as a go-to Republican for the five boroughs, taking over a role long held by Rep. Vito Fossella of Staten Island, who is leaving his seat following a drunken driving arrest and the public disclosure of an extramarital affair.

Mr. Fossella remains in office, but his profile has diminished in the weeks since his scandal erupted and he announced he would not seek re-election. Republicans in the 13th District struggled mightily to find a candidate to run in his stead, while Democrats are aggressively targeting the seat.

City officials and Democrats in the congressional delegation say they are increasingly turning to Mr. King to advocate on behalf of New York City with the Bush administration, a role that could expand if Democrats gain the Fossella seat but Republicans retain the White House next year. Mr. King has also become more important statewide, if only because the Republican ranks fell in 2006 and could shrink further in a political climate that favors Democrats this fall.

“He realizes now that he is one of the senior members of the New York delegation on the Republican side, and as a result there’s additional responsibility for him and he’s ready to stand up,” a Democrat of Queens, Rep. Gregory Meeks, said. When Governor Paterson invited the state congressional delegation to a dinner in Albany shortly after he took over for Eliot Spitzer, Mr. King was the only Republican who showed up, Mr. Meeks said.

The shift from Mr. Fossella to Mr. King has been gradual but clear. Mr. King said aides to Mr. Bloomberg approached him at a delegation dinner the mayor held at Gracie Mansion shortly before Mr. Fossella announced he would step aside in May. That they would be relying more on him, Mr. King said, “was implied in everything that was said.”

“There was no dramatic announcement or anything like that. It wasn’t like the Magna Carta being signed,” he said. He has also joined Mr. Fossella in meetings with Bush administration officials and has stayed in regular contact with members of his staff. “There was absolute cooperation there. Vito couldn’t be better,” Mr. King said of Mr. Fossella, a close friend.

An expanded portfolio of city issues for Mr. King makes logical sense given his seniority and the proximity of his Nassau County district, but it also has a clear political upside. The congressman is weighing a bid for governor in 2010, and a greater profile in the five boroughs and upstate will be key to his chances.

Mr. King said he would not make a decision about a gubernatorial campaign until next year but that some party activists have approached him about running. “If serious people mention it to me, I have to look at it seriously,” he said.

Complicating the choice is speculation that either Mr. Bloomberg or Mayor Giuliani — or both — could seek the Statehouse on the Republican line. Mr. King is close to both men and particularly to Mr. Giuliani, whom he endorsed in the Republican presidential primary. He all but ruled out a bid for himself if the former mayor enters the race. “If Rudy runs, that’s about as categorical a ‘no’ as I can imagine right now,” Mr. King said, adding that the same was likely true for a Bloomberg candidacy. “I wouldn’t divide the party to run in a race where I’d have a very difficult time winning anyway.”

Mr. King draws high praise from the Bloomberg administration and several members of the delegation as a straight shooter who focuses on issues and works well with Democrats. “He shatters the line between Republican and Democrat,” the dean of the state’s delegation, Rep. Charles Rangel of Harlem, said in an interview.

Through a spokesman, Senator Clinton called Mr. King “a tireless partner” in the push for more federal funding for New York. The two have had a cordial relationship since the 1990s, when Mr. King was one of few House Republicans to oppose the impeachment of her husband.

Mr. Bloomberg, in a statement to The New York Sun, said Mr. King has “stood with the city 100%” in its push to secure more homeland security funding and had been “a clear voice of reason on issues ranging from 9/11 health care to illegal guns.” “I’ve always believed that getting big things done requires reaching across the aisle, and Pete does it as well as anyone,” the mayor said.

There have been areas of tension, however. Mr. King has sided with conservatives in his party on immigration, opposing what he derides as “amnesty” legislation that would create a path to citizenship for illegal immigrants. Mr. Bloomberg favored a path to citizenship, arguing that the city would suffer from a crackdown on immigration and accusing hard-liners on the issue of xenophobia.

Early in his career in Congress, Mr. King had been known for his staunch defense of the Irish Republican Army and its political arm, Sinn Fein. He cooled to the IRA after September 11, however, and in 2005 he called for it to disband. He also drew notice during the 1990s after he led the charge against the awarding of federal housing contracts to a security company affiliated with the Nation of Islam.

Mr. King has toed a partisan line on other issues. In the days following Mr. Fossella’s arrest and the news about his affair and a child out of wedlock, Mr. King urged restraint amid calls for Mr. Fossella to resign. Just two months earlier, however, Mr. King had been on television calling for Mr. Spitzer, a Democrat, to step down within hours of the disclosure that he had been linked to a prostitution ring.

Still, city Democrats say they are confident they can work with Mr. King — as they have in the past — on aid for ground zero workers and other issues, such as transportation and homeland security funding, that cleave along geographic rather than party lines. “He’s the kind of guy that will always speak his mind, so you know where he stands,” Mr. Meeks said. “He is a stand-up, straight-up guy.”