Representative Thomas Suozzi, a Long Island Democrat, announced on Monday that he would enter the race for governor of New York, broadening the field of candidates running against the incumbent, Kathy Hochul, and becoming the first Democrat to take direct aim at her support among moderate suburban voters.

Mr. Suozzi, who has most recently focused on federal negotiations over raising a cap on state and local tax deductions, has positioned himself as a vocal centrist who is quick to lash what he casts as the excesses of his party’s left wing.

His decision to run for governor, which he made official at a virtual news conference, will intensify and complicate the battle for moderate voters in one of the nation’s marquee Democratic primary contests next year.

“I don’t believe it’s about going to the far left or to the far right,” said Mr. Suozzi, who outlined an agenda that includes lower property and income taxes, robust efforts to fight crime and reduce homelessness, and a focus on combating the coronavirus pandemic and its effects on the economy.

“I’ll work with anybody,” he said. “It’s not about being politically correct. It’s about doing the correct thing to actually help people.”

Mr. Suozzi, a former Nassau County executive who is billing himself as a “common-sense Democrat,” could cut into parts of the coalition Ms. Hochul is seeking to assemble on Long Island and in suburbs around the state.

Though Mr. Suozzi is a strong fund-raiser, he nevertheless will face steep challenges in a statewide Democratic primary.

While early polling has limited value ahead of a primary slated for next June, he was in the single digits in a recent survey. Ms. Hochul, the state’s first female governor, has consistently led the field in early polls and has an overwhelming head start in fund-raising and endorsements.

Other candidates also have the kind of history-making potential that Mr. Suozzi, a white man, does not — most notably Attorney General Letitia James, who could be the first Black female governor in the country should she win. She and Ms. Hochul are widely seen as the two most formidable candidates at this stage of the race.

Congressional Democrats are expected to face a brutally challenging environment in next year’s midterm elections, but Mr. Suozzi insisted that had no bearing on his decision to run.

“I feel like this whole, you know, left-right extremist thing in our country is killing our country and it’s killing our state,” he said. “I don’t think I could sit on the sidelines and watch what’s happening, watch what’s happening to our state, and not be engaged in an effort to try and bring it forward.”

Mr. Suozzi’s candidacy for governor could risk the Democrats’ hold on his largely suburban House seat at a time when they are battling nationally to retain control of the chamber.

Without a popular incumbent there to defend it, Republicans would likely make the seat a top pickup target in New York. Democrats could find themselves spending large sums to defend the seat or be forced to give it greater consideration during the once-in-a-decade redistricting process.

Diverting more Democratic voters to the district could in turn complicate the party’s efforts to use the redistricting process to seize one or two more House seats on Long Island.

“Tom Suozzi is making the smart decision to quit Congress rather than lose in 2022,” said Camille Gallo, a spokeswoman for the National Republican Congressional Committee.

On the Democratic side, Melanie D’Arrigo, who describes herself as a health care advocate and community organizer, has said she is running; she lost to Mr. Suozzi in a primary last year.

A number of others are thought to be eying the seat, including Robert Zimmerman, a Long Island businessman and a Democratic national committeeman; Josh Lafazan, the youngest person ever elected to the Nassau County Legislature; and Assemblyman Edward Braunstein.

Two Republicans have also opened federal campaign accounts, including George Santos, who appears to be raising funds at a steady clip.

In theory, Mr. Suozzi still has time to reverse his decision before the state’s filing deadline for candidates next spring. But elections experts say that Mr. Suozzi could not seek two offices on the same ballot, and national Democrats say they now consider the House race an open seat.

Asked if he could rule out being a candidate for Congress under any circumstances next year, Mr. Suozzi replied, “I’m running for governor of New York State.”

Mr. Suozzi began discussing with allies whether to run for governor at least as early as this summer, after Ms. James’s office released a damning report detailing accusations of sexual harassment by then-Gov. Andrew M. Cuomo, according to people involved in the talks.

But he has long been interested in the job.

He ran for governor in 2006, and was trounced in the Democratic primary by Eliot Spitzer, who would later resign from the governorship in disgrace.

In that race, Mr. Suozzi ran on a message of managerial competence, a theme he is reprising, this time by also citing his federal experience. His campaign slogan in other races has been, “Suozzi gets it done.” This time around, there is a slight modification: “a common-sense Democrat who gets things done.”

The congressman will enter an increasingly volatile field shaped by ideology, identity and geography. The growing number of candidates has injected a fresh measure of uncertainty into a race that will offer a major test of the direction of the Democratic Party.

Ms. James, of Brooklyn, is seeking to assemble a coalition rooted in Black and Latino voters of varying political leanings, and support from white progressives.

But she faces geographic and some ideological competition from Jumaane D. Williams, a fellow Brooklynite who hopes to emerge as the left-wing standard-bearer, and would be the state’s second Black governor.

Bill de Blasio, New York City’s mayor and another Brooklyn resident, has indicated that he is also moving toward a bid, despite facing deep skepticism among many Democratic leaders.

Significant questions around the field remain, including whether and how Mr. Cuomo, who resigned in disgrace, may deploy his remaining war chest to engage in the race.

After Democrats sustained major losses on Long Island and around the country earlier this month, Mr. Suozzi is plainly betting that voters worried about next year’s general election will be more receptive to a relative centrist.

Recent filings show he is sitting on more than $3 million in his federal campaign account that could likely be rolled over to a gubernatorial bid to make that case.

But he faces fierce competition for the moderate mantle — from Ms. Hochul and, to some degree, potentially Ms. James — even as he runs in a primary in which New York’s vibrant progressive-left movement could be powerfully influential.

Jay Jacobs, the chairman of the New York State Democrats and of the party in Nassau County, chaired Mr. Suozzi’s 2006 bid. He has endorsed Ms. Hochul and said he recently sent Mr. Suozzi a memo with his assessment of the race.

“I don’t see any path for him to win and his entry into the race can only cost Kathy Hochul some votes,” he said.

Mr. Suozzi, for his part, said he planned to build a broad coalition, and cast himself as a risk-taker.

“I’ve shown that I’m willing to risk it all, sometimes with great success and sometimes crashing and burning,” he said. “But I’m willing to risk it all when I see something that needs to be done, and I’m willing to take those chances.”