Former MTA chief Joe Lhota confirmed the city's worst-kept political secret Monday when he said he will file papers at the end of the week launching his mayoral run.

Lhota, a former deputy mayor under Rudy Giuliani, told members of the New York Building Congress trade group during its annual membership meeting his decision to leave the MTA last month was tough, but he aims to help the city move forward.

"I would not have left the MTA, a job and position that I loved, if I was not going to run for mayor of New York," he told the 500 attendees.

Lhota touted the MTA's work to keep the trains safe during superstorm Sandy and its round-the-clock efforts to get the system running efficiently in the days after the storm.

Although the candidate, who likely will run as a Republican, refused to talk more about his campaign before filing, political experts said he'll take the right first steps in his bid.

Andrew Moesel, a political consultant for Sheinkopf Communications, said Lhota has to give voters an idea of who he is and raise money for the primary and general elections.

"He's got a lot to do and not a lot of time," Moesel said.

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Moesel said Lhota's speech at the luncheon would help him play catch-up in fundraising with other potential candidates such as City Council Speaker Christine Quinn, Public Advocate Bill de Blasio, City Comptroller John Liu and former city Comptroller Bill Thompson.

Lhota, a Long Island native who lives in Brooklyn, served in many high-level positions before beginning his nine-month term at the MTA, including being executive vice president of administration for the Madison Square Garden Co.

The Dolan family owns controlling interests in Madison Square Garden and Cablevision. Cablevision owns Newsday.

Giuliani has said he thinks Lhota would be a good mayor, and that support may be a double-edged sword, experts said.

Joe Reubens, a political director at the political consulting firm the Parkside Group, said many voters are still burned out by the former mayor, especially with what he called Giuliani's right-wing remarks during his presidential runs.

"While it may be helpful in a Republican primary, I don't think it's positive in the general election," Reubens said.

Despite his GOP ties, the candidate expressed support for same-sex marriage and the legalization of marijuana.