The food workers also have 19,000 members in Wisconsin, which holds a primary Tuesday.
The union is made up of supermarket workers and meatpackers, with 40 percent of the membership under 30 years old. Obama has been doing especially well among young voters.
"Senator Obama's message of changing hope into reality has inspired our members, particularly our young members, across the country," Food Workers President Joe Hansen said in a statement in which he also lauded Clinton's tireless efforts on behalf of workers.
"While both Senators Hillary Clinton and Barack Obama have a vision to change America, we believe that Senator Obama is the best candidate to build a movement to unite our country that will deliver the type of change that is needed for good jobs, affordable health care, retirement security and worker safety," he said. He called Obama "the candidate of the American dream."
In 2004, the union initially backed Rep. Dick Gephardt, D-Mo., a labor favorite who dropped out of the presidential race after a poor showing in Iowa. The union later endorsed Democratic nominee Sen. John Kerry.
Separately, the Service Employees International Union executive board planned a conference call Thursday night to consider an Obama endorsement, according to a person familiar with the plans. This person was not authorized to speak publicly and so requested anonymity.
The international union had stayed out of the primary race until now, because its leaders were divided over who to endorse after an especially strong courting by former Sen. John Edwards, who dropped out of the race just before Super Tuesday. The 1.8-million member union allowed its state affiliates to make endorsements, and most of them backed Edwards.
