Denise O’Donnell is taking a pass on a second attempt at the AG’s office this year and will back Nassau County DA Katheen Rice – now the lone woman in the field – instead, a source familiar with her plans confirmed.

The Buffalo Democrat will formally announce her decision today at a press conference at 2:30 p.m. on the City Hall steps in Manhattan.

O’Donnell resigned her post as Gov. David Paterson’s public safety czar in February in the wake of the David Johnson domestic violence scandal, which theoretically freed her up to raise campaign cash and woo Democratic leaders.

But she would have had to work very hard to catch up with her primary opponents, who have been campaigning – albeit quietly, since AG Andrew Cuomo has yet to formally say he’s not seeking re-election – for months now.

A former US attorney for the Western District of NY during the Clinton administration, O’Donnell briefly ran for AG in 2006 and was widely believed to be one of Eliot Spitzer’s preferred successors. (He also liked Mark Green quite a bit, due to the financial largesse of Green’s real estate developer brother, Stephen. Basically, Spitzer liked anyone but the hyper-ambitious governor-in-waiting Cuomo for AG that year).

O’Donnell dropped out of the running after failing to get onto the ballot at the state Democratic convention (held in her home town), and later joined the Spitzer administration.

Rice is speculated to be Cuomo’s preferred candidate because she would bring balance to the ticket as a woman and suburbanite. She routinely seeks to link herself to Cuomo and recently said the two of them are “like-minded.”

Rice also refers to herself as an “outsider” and a “career prosecutor,” but her crime-fighting record has been under fire by one of her AG primary opponents, Sen. Eric Schneiderman, whose campaign has noted that crime in Nassau County has actually risen on Rice’s watch.

Former city Comptroller Liz Holtzman has also been eyeing a potential AG run, which would throw a wrench in Rice’s ticket-balancing argument.

But she has yet to formally enter the race and took her name out of consideration when she was nominated at the DRC straw poll in Niagara Falls, which was won by former state Insurance Superintendent Eric Dinallo. (Rice came in second).