"We had a wonderful dilemma," D. Taylor, the union's secretary treasurer, said in a news conference. "It's been a very difficult decision. We understand we are going against the Democratic power establishment [...] we are used to being underdogs," he added.
The Culinary Workers Union, Local 226, is part of the parent union Unite Here, which has 450,000 members. The endorsement is Obama's first from a major national union, according to the Associated Press. He also picked up an endorsement from the 15,000-member Service Employees International Union in Nevada Tuesday night.
The decision is a disappointment for the Clinton campaign, which has been leading in the polls in Nevada.
"It's a little bit of a surprise," David Damore, an associate professor of political science at the University of Nevada at Las Vegas, told the AP. "A lot of people expected, especially after Clinton's turnaround in New Hampshire yesterday, that they would go for the establishment candidate."
All three Democratic front-runners had campaigned for Culinary Workers Union endorsement, which is expected to be able to bring out thousands of caucus-goers. Since November, the union has been training its members on how to caucus as well as going door-to-door to register its members, the Politico reported.
This turnout effort is seen as particularly important because the campaigns expect overall turnout for the Nevada caucuses to be low -- the state had never played a significant role in choosing a nominee before this year. Democratic state party officials say they expect a turnout of only about 10 percent of registered Democrats, or about 45,000 people, the AP reported.
Some analysts, though, have questioned whether the union's decision to delay its endorsement until after the New Hampshire primary will diminish its power, as the union now has only 10 days to bring its members out for Obama.
In recent days, it had been unclear which candidate the union would pick.
In the immediate aftermath of Obama's victory in the Iowa caucuses Jan. 3, some media outlets reported that the union's endorsement was his, but by Tuesday night's Clinton victory in the New Hampshire primary, there was still no official announcement.
The Politico reported that Rory Reid, chairman of Clinton's Nevada campaign and son of Senate Majority Leader Harry Reid (D-Nev.), said the campaign was continuing to make its case to the union.
Meanwhile, former North Carolina Sen. John Edwards had hoped that his strong relationship with unions, including the leadership of Unite Here, would sway the group to his candidacy.
Obama will travel to Nevada on Friday for a rally and to officially accept the endorsement, according to his campaign staff.