Stern said that in the months since union leaders met with several Democratic candidates last fall, "the excitement has been building and building for Obama."
The politically active union represents workers in health care, building services and other industries. It has donated more than $25 million to candidates in the past two decades, most of it to Democrats.
For Obama, the endorsement offers a chance to increase support in the primary states that are scheduled to vote in the next few weeks, particularly Ohio and Texas on March 4 and Pennsylvania on April 22.
On Thursday, Obama collected the support of the United Food and Commercial Workers, a politically active union with significant membership in the upcoming Democratic battlegrounds. The 1.3-million member UFCW has 69,000 members in Ohio and another 26,000 in Texas. 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.
The SEIU delayed a national endorsement for months after inviting Obama, Clinton and other Democratic candidates to speak to its members in Washington.
In the interim, several state affiliates swung behind candidates, many of them choosing former Sen. John Edwards.
Edwards dropped out of the race just before the Super Tuesday primaries and caucuses on Feb. 5, leaving the field to Obama and Clinton.
Sarah Swisher, a superdelegate and member of the SEIU from Iowa City, had committed to Edwards. After he quit the race, she switched to Sen. Hillary Rodham Clinton, but she changed her mind again after her union endorsed Obama. "That will be kind of cool," Swisher said. "I will have supported all three."
Stern said in a telephone interview that siding against Clinton was not an easy thing to do.
"It is hard because Senator Clinton is both a great senator and a good friend to the SEIU," he told The Associated Press shortly after the announcement.
Stern said the union waited this long to endorse a candidate because there was a lot of support for John Edwards and "we didn't want to look like we were bouncing between candidates." The Obama endorsement came after careful consideration, he said, and polling of the membership.
"Our members and leaders really feel that American needs profound change and something that engages the next generation," Stern said, adding that Obama fits the bill.
Last October, the union said it would hold off on an endorsement in the primary because of divisions that had been apparent among SEIU supporters of Edwards, Clinton and Obama.
"Given the importance of this election, we are encouraging members and leaders to act on their passion for the candidates and get involved on a statewide basis," Stern said at the time.
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Associated Press writers Ron Fournier in Washington and Nedra
Pickler in Milwaukee contributed to this report.
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