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by John McCormick and Steve Franklin, updated

The powerful Service Employees International Union plans to endorse the presidential bid of Sen. Barack Obama, a source with knowledge of the agreement said this afternoon.

It remains unclear when or where the endorsement would formally be made.

The United Food and Commercial Workers, a politically active union with 1.3 million members, also announced an Obama endorsement this afternoon. The union’s members work primarily in food markets and meatpacking, food processing and poultry industries.

Union leaders said Obama’s string of eight state primary and caucus wins since Super Tuesday were a factor in making the endorsement.

“There is an excitement within our union — about 40 percent of our members are less than 30 years old — and his appeal to that group has translated to our own union,” said Joe Hansen, president of the UFCW, a one-time Chicago butcher who rose up through the union’s ranks.

On the SEIU endorsement, Hansen said, “I think they will announce tonight or tomorrow.”

The endorsements give Obama a major boost in advance of nominating contests in heavily blue-collar states like Wisconsin, Ohio and Texas.

Obama is taking a day off from the campaign trail today to rest and take his wife, Michelle, out for a Valentine’s Day dinner in Chicago. He is expected to return to Wisconsin Friday morning, in advance of Tuesday’s primary there.

The national SEIU, with at least 1.5 million members, had stayed away from endorsing any of the Democratic candidates. Its state affiliates were allowed to make endorsements and many backed John Edwards, the former North Carolina senator who recently left the Democratic nomination race.