With the Iowa caucuses just over two weeks away, the Illinois senator is the only top-tier Democratic candidate who has not been endorsed by a national union.
Such endorsements are prized because of the manpower, money and attention they can garner for candidates in the early voting states such as Iowa and Nevada. For example, the political arm of the American Federation of State, County and Municipal Employees, which has endorsed New York Sen., spent $250,000 to air television ads in Iowa urging her victory.
And Clinton has nine other well-heeled national unions in her camp, while former North Carolina Sen.has four national union endorsements.
Obama, meanwhile, has had to make do with city chapters like the Correction Officers' Benevolent Association of New York City and Illinois state chapters of the American Federation of Teachers, AFSCME and the Service Employees International Union. Other state chapters of the service employees union behind Obama include those in Indiana, Wisconsin, Missouri and Kansas.
His politics aren't the problem, analysts and supporters say.
Tom Balanoff, president of the SEIU Illinois State Council, said Obama's voting record is sound, with votes against trade deals like the Central America Free Trade Agreement and support for issues such as the Employee Free Choice Act. "We know that he's the real thing," Balanoff said.
Obama himself touts a longtime union record. "I've been consistent. You can't say that about the other two major candidates," Obama told a regional convention of the United Auto Workers in Iowa. "When a candidate says he opposes right-to-work laws or trade rules that hurt workers today, ask him where he's been before. Because America needs a president who will fight for you when it's hard, and not when it's politically convenient."
But even supporters like Balanoff said that Obama's relatively short time in the national spotlight may be working against him. While labor officials in the Midwest have known Obama for years, he's only been a U.S. senator since January 2005.
Obama "hasn't had the exposure that Hillary Clinton has - everybody knows Hillary - and John Edwards, who has run for president and run for vice president and has done a lot of work with unions," Balanoff said.
Added Paul Clark, head of the department of labor studies and employment relations at Penn State University: "Compared to the other candidates, he's a latecomer. Clinton and Edwards have a much longer relationship with the unions. ... I just think he had a lot of ground to make up."
The lack of major union endorsements early in the race may not be crucial to winning the Democratic nominations. Only one major union endorsed the 2004 Democratic nominee, Sen. John Kerry, D-Mass., before the primaries and caucuses - the International Association of Fire Fighters.
Despite supporting other candidates in the nominations battle, organized labor coalesced behind Kerry in his unsuccessful race against President Bush. Unions are expected to support whoever wins the 2008 Democratic nomination.
Robert Bruno, a professor at the Institute of Labor and Industrial Relations at the University of Illinois at Chicago, said Obama may be a victim of his desire to be a "transformative politician" - someone who appeals to people in all demographics.
Obama's been trying "to avoid the claim he's just like any other politician, so he doesn't come off as a real advocate," Bruno said. "But I think the labor movement wants an advocate, because you're talking significant resources and investment."
Organized labor put more than $60 million into the 2004 national elections, a figure that likely will be eclipsed by the end of the 2008 elections.
The fact that he's black has nothing to do with Obama's lack of union endorsements, said Richard Hurd, professor of labor studies at Cornell University. Almost 80 percent of union members in the United States are white.
"Forty years ago, a totally different story. Twenty years ago, a mixed bag. Today, I don't there's any chance it has anything to do with it," Hurd said.
In fact, Hurd thinks race could help Obama. Almost 15 percent of black workers are union members, compared with almost 12 percent of white workers and 10 percent of Hispanic workers.
"Unions actually have more success organizing blacks and Latinos than they do white people," Hurd said. "That would be something that would tempt them into endorsing Obama, because most unions have a very high priority on recruiting new members. So I think if race came into play here, it would be in Obama's favor rather than against him."