Gillibrand is running for her first full six-year term and the action by state Democratic committee members was expected. Gillibrand told the crowd she would fight for all New Yorkers.
"I have a goal -- we want to make sure all of our voices are being heard," Gillibrand said.
New York Conservative Party leaders made their choice in Manhattan late.
Conservative Party chairman Mike Long had spoken in favor of Wendy Long, who's in a Republican primary with U.S. Rep. Robert Turner (R-Rockaway Point) and Nassau County Comptroller George Maragos. The Conservative line is considered vital to Republicans running for statewide office in New York. The two Longs aren't related.
Wendy Long said her campaign would "bring people together on the issues that matter the most to New Yorkers, jobs and the economy."
Gillibrand, a former upstate congresswoman, was picked in 2009 to replace Hillary Rodham Clinton, who became secretary of state. Gillibrand won election in 2010 to fill the remainder of the term, which ends this year.
The lesser-known candidates seeking to challenge her are expected to face an uphill battle.
Still, Republicans, who met Friday in Rochester for their convention, said Gillibrand remains vulnerable after three years in the Senate. Republican voters will choose among the three candidates in a June 26 primary vote.