Obama's name is the only one on the ballot sent to the AFL-CIO's 56 unions.
The AFL-CIO's endorsement is virtually certain now that Obama is the presumptive Democratic presidential nominee. It has already started its campaign against Republican presidential candidate John McCain, and its biggest rival, the Change to Win labor organization, already has endorsed the Democratic senator from Illinois.
A strong AFL-CIO endorsement could help Obama with blue-collar workers and union members in industrial states such as Pennsylvania, Ohio and Michigan. The AFL-CIO expects to spend about $200 million on the presidential and congressional elections, much of it on Democrats.
The AFL-CIO stayed neutral during the Democratic primary, allowing its unions to make individual endorsements. More than a dozen threw their support to Obama rivals John Edwards, Christopher Dodd and Hillary Rodham Clinton.
But Obama got an enthusiastic reception from the AFL-CIO's leadership last week when he met with them at their headquarters a block from the White House - even from union leaders who formerly supported Clinton, who came in second behind Obama in the primary.
Clinton dropped out of the race earlier this month and endorsed Obama.
Officials representing two-thirds of the AFL-CIO's 10.5 million members must support Obama for him to get the endorsement.