And for the past week the union has been using its 22-line phone bank to round up Clinton supporters for the caucuses.
The endorsement could be a big boost for Obama as he heads into the Democratic Party's precinct caucuses on Saturday.
Congressman Adam Smith of Tacoma, Obama's state chairman, announced the endorsement this afternoon in a conference call with reporters.
"Nobody makes a bigger difference in organizing campaigns in this state than SEIU," Smith said.
The union, which has about 100,000 members statewide, had previously endorsed former Sen. John Edwards. But after a dismal showing in the early contests, Edwards dropped out of the race last week.
Leaders from the SEIU's five major locals in Washington met Tuesday to decide whether to throw its support to Obama or his Democratic rival, Sen. Hillary Rodham Clinton.
"It ended up being a unanimous decision by all of our locals," said Adam Glickman, spokesman for SEIU Local 775. "Obviously, there are two very strong candidates, both of whom are vast improvements over the current office holder."
Glickman said the union would be making tens of thousands of calls to its members over the next three days to urge support for Obama. He said the calls also are intended to make sure members understand that — in order to have a say in choosing Democratic delegates — they have to participate in the caucuses.
The Democratic Party is allocating all of its delegates through the caucuses and is ignoring the results of the state's Feb. 19 presidential primary.
Clinton, meanwhile, has been getting a lot of support from the International Association of Machinists and Aerospace Workers, which represents about 25,000 workers in this state.
In fact, the Clinton campaign is using the union's offices in South Park as a makeshift campaign headquarters.