In a statement, Stephanie Schriock, Emily's List president, called Buono "the perfect candidate" to become New Jersey's first female Democratic governor.

"The EMILY’s List community — now two million members strong — is ready to unseat the aggressively anti-woman Governor Chris Christie, and replace him with a smart and pragmatic fighter who will expand opportunity for women and families," Schriock said.

The endorsement comes the day after Emily's List put New Jersey Gov. Chris Christie (R ) "on notice," a designation for the group's top targets.

But Buono enters the race the distinct underdog and relatively unknown in the state. Christie maintains high favorability ratings in New Jersey, with nearly two-thirds of state residents approving of his job performance and more than 70 percent saying he deserves reelection in a recent poll.

Democrats maintain that he's vulnerable, pointing to what they consider a sluggish record on job creation, but running against Christie was enough of a challenge to persuade New Jersey Democrats' top choice to run, Newark Mayor Cory Booker, to pursue retiring Sen. Frank Lautenberg's (D) seat instead.

The Emily's List endorsement will come with fundraising help, and potentially ad expenditures from the super PAC affiliated with the group, Women Vote.

--This story was updated at 8:22 a.m.