The sought-after endorsement would be Obama's largest from organized labor, and give him a powerful boost against rival Sen. Hillary Rodham Clinton in the March 4 presidential primaries in Ohio and Texas.
The 1.8 million-member union is likely to endorse Obama on Friday, the union officials said, speaking on condition of anonymity because they were not authorized to speak to the media.
Union leaders decided after a conference call Thursday night to go with Obama.
The officials who spoke anonymously cautioned that the union was still voting, but Obama was "99 percent" likely to get the endorsement, one of the insiders said.
The union had announced earlier Thursday that "President Andy Stern and Secretary-Treasurer Anna Burger will discuss a major political announcement with reporters" on Friday.
Separately Thursday, Obama also won the backing 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.
With an SEIU endorsement and the United Food and Commercial Workers' backing, Obama would only need to pick up one more union endorsement to be eligible to collect the Change to Win labor federation's support. There are seven unions in the federation, and it would take endorsements from at least four of them to make the federation consider a joint endorsement.
Obama was endorsed in January by UNITE HERE, which along with SEIU and the United Food and Commercial Workers, would give him three of the seven member unions. The Teamsters, the Laborers' International Union of North America, and the United Brotherhood of Carpenters and Joiners of America have yet to endorse a candidate.
The seventh union, the United Farm Workers, endorsed Clinton in January.
Obama also was endorsed earlier this month by the Transport Workers Union and the National Weather Service Employees Organization.