Joseph Lhota, who helped guide the city's recovery from the Sept. 11 attacks and superstorm Sandy, is exiting his role as Metropolitan Transportation Authority chairman and chief executive and training his sights on Gracie Mansion.

Lhota, a Republican, sent the governor a resignation letter Wednesday, so he can explore joining the field seeking to succeed New York City Mayor Michael Bloomberg, whose term expires in November.

The resignation is effective at the end of the month. Lhota, 58, said he will discuss a "potential candidacy" with his family and decide whether to run in January. The Bronx-born son of a retired NYPD lieutenant, he is married to Tamra Lhota, a Republican fundraiser. Both have close ties to former Mayor Rudy Giuliani.

Lhota was appointed to the MTA post last Jan. 4, at Gov. Andrew Cuomo's recommendation. His term was to expire in June 2015. By law, Lhota cannot run for mayor while holding the MTA chairmanship.

"This will be a life-defining decision and one that I will be seriously considering in the upcoming weeks," Lhota said at a news conference Monday, after presiding over an MTA board vote to hike fares on buses, subways and rails for the fourth time in five years.

Lhota said he's not turning his back on the MTA and hopes to be supportive in the future.

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"My decision is bittersweet. I never expected this to happen," he said.

Lhota, a former investment banker, made a name for himself in city politics while working under Giuliani, first as city budget director and later as deputy mayor of operations. Giuliani called Lhota his right-hand man in the aftermath of the Sept. 11, 2001, terror attacks.

"I know how difficult that job is," Lhota said about the mayorship at the news conference. He deflected questions about gun control and same-sex marriage -- the sorts of issues he would be asked about on the campaign trail. "I also know how challenging that job is. And I also know how important that job is in the lives of all New Yorkers."

Talk of Lhota's interest in running for mayor grew louder in the days after Sandy, when he won praise for his hands-on approach in leading the nation's largest public transportation agency through the worst natural disaster it had seen.

Lhota lives in Brooklyn Heights. He grew up in Lindenhurst and graduated from St. John the Baptist Diocesan High School in West Islip. He is a graduate of Georgetown University and Harvard Business School.

He and his wife have an adult daughter, Kathryn.

Before joining the Giuliani administration, Lhota was executive vice president of administration for The Madison Square Garden Co., which, like Newsday, is owned by Cablevision Systems Corp.

Gene Russianoff, a spokesman for the Straphangers Campaign public transport advocacy group, said he was a little surprised by Lhota's interest in a mayoral bid.

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"I thought Joe Lhota had been on a roll," he said. "He did a great job during his (tenure), especially during Sandy."

John Samuelsen, president of TWU Local 100, the largest union for city transit workers, said Lhota's reputation benefitted from the efforts of the workers who restored the system after the hurricane, "yet he leaves without having settled a fair contract with the workers who propelled him to this level."

With Lhota gone, New York City Transit president Thomas F. Prendergast will work double duty, taking over day-to-day operations of the MTA until a new chief is appointed. Board member Fernando Ferrer was named vice chairman and will serve as acting chairman when the resignation takes effect.

Ferrer will become the agency's first Latino chairman. In 2005, he became the first Latino to win the Democratic nomination for New York City mayor. The former city councilman and Bronx Borough president said he has no designs on replacing Lhota.

Asked if he wanted the job, Ferrer said, "No."

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He described his role in the coming weeks as "holding down the fort while we get a permanent chairman."

Rumored contenders to replace Bloomberg after three terms include City Council Speaker Christine Quinn, Public Advocate Bill de Blasio and City Comptroller John Liu, all Democrats. The only announced candidates so far are Manhattan Media publisher Tom Allon, who is running as a Republican, and former city comptroller Bill Thompson, a Democrat who lost to Bloomberg in the 2009 election.

A Quinnipiac University poll released in November found Lhota losing 60-9 percent to an unnamed Democratic candidate. The poll found Quinn leading the Democratic primary field with 32 percent of the vote.

Craig Eaton, Brooklyn Republican chairman, said he is still "very bullish" on another candidate -- Adolfo Carrion Jr., the former Bronx borough president -- but added that if Lhota runs, "we will take a serious, hard look" at him.

Queens County Republican chairman Phil Ragusa said he still favors John Catsimatidis, the grocery magnate who owns the Gristedes supermarket chain. Catsimatidis has not declared. Ragusa offered that Lhota "would probably be a good candidate." Manhattan GOP chairman Dan Isaacs declined to comment, saying he hadn't spoken to Lhota.

With Yancey Roy, Marc Beja and Ivan Pereira