By Ellen Wulfhorst

NEW YORK (Reuters) - New York City Council Speaker Christine Quinn, considered a front-runner to be New York's next mayor, officially launched her candidacy on Sunday for the job that would make her the city's first female and first openly gay mayor.

The Democratic Quinn, 46, made her announcement on Twitter, saying: "It's official. I'm running for mayor to fight for the middle class."

In a video linked to her tweet, Quinn talked about the importance of safe streets, improved schools, affordable child care and housing and the creation of good-paying jobs. (http://www.quinnfornewyork.com/announcement)

Quinn has the support in the November election of Mayor Michael Bloomberg, according to his aides. Bloomberg's 12-year stint in City Hall comes to a close at the end of the year due to term limits.

Other contenders in the mayoral race, either officially or likely, include Public Advocate Bill de Blasio, former Comptroller Bill Thompson, Comptroller John Liu and former Metropolitan Transportation Authority Chairman Joseph Lhota.

Quinn, a former tenant organizer, has been a member of the City Council since 1999 and was elected Speaker in 2006.

Launching a "walk and talk" tour of the city's myriad neighborhoods, in the video Quinn urged New Yorkers to "put those sensible shoes on and come on out with us."

In her video, Quinn said she loved New Yorkers for "how colorful they are, how in the best of all possible ways crazy they are..."

Quinn is married to Kim Catullo, an attorney.

(Editing by Bill Trott)