New York City took a step closer to having its first female and openly gay mayor on Sunday with the formal announcement that strongly tipped Democrat Christine Quinn will run for the post.

The city council speaker and veteran of New York politics confirmed her intention via a tweet and released a video in which she vowed to fight for middle- and working-class New Yorkers.

An early favourite to get the Democratic nomination for the mayoralty, she is also expected to get the backing of Republican-turned-independent mayor Michael Bloomberg.

Her closeness to the incumbent has been criticised by those on the left of her party who claim that she has betrayed her principles on occasion to advance politically.

Notably, critics point to her backing of Bloomberg's objection to a plan that would have forced businesses employing at least five workers to provide paid sick leave. She was also criticised for helping Bloomberg get the green light to extend term limits so that he could run for a third time in 2008.

But Bloomberg's backing, and the business leaders that support him, would provide a major boost in her bid to become the mayor of the US's most populous city when voters go to the poll later this year.

A former tenant organiser and director of a gay and lesbian advocacy group, Quinn, 46, has been on the city council since 1999 and its leader since 2006. A Quinnipiac University poll late last month gave her 37% of the Democratic vote, while her opponents each got less than 15%.

But despite registered Democrats outnumbering Republicans in the city by a factor of six to one, it hasn't translated into Democratic success in a mayoral race since 1989. Nonetheless Quinn also has a healthy lead in a hypothetical match-up with the leading Republican contender, former city transport chief Joseph Lhota.

In a video accompanying Sunday's announcement, Quinn pushed her credentials while pushing her case as a defender of the middle and working classes.

"I'm about keeping New York City a place for the middle class to live and grow and a place that is going to help those hard working people get into the middle class," she said. "I'm not about talking and finger-pointing and complaining; I'm about getting things done."

If successful, a Quinn mayoralty would be a historic first for the city in two respects: she would become the only woman to have held the post and also first to be openly gay.