(CNN) - One of the nation’s strongest unions, the Service Employees International Union, endorsed Barack Obama for president Friday.
The union’s support comes with a massive grassroots voter outreach effort that is a huge force in Democratic politics. That operation will now work on Obama’s behalf nationwide, including the major primary state battlegrounds of Ohio, Texas and Pennsylvania.
In a conference call with reporters, SEIU Secretary-Treasurer Anna Burger said the union would immediately begin help mobilize its members and resources on the ground in Wisconsin, which votes on Tuesday.
“When we do an endorsement, we take it seriously,” said Burger.
Earlier in the cycle, John Edwards had wooed many SEIU locals, which endorsed his presidential bid. But the national leadership resisted pressure to back a candidate, saying they would remain neutral through the primary season.
Three sources familiar with the deliberations told CNN’s John King Thursday that union leaders had met via conference call that day to deliberate on the endorsement issue – and that Obama had emerged as the overwhelming consensus choice of state and national leadership.
“We have an enormous amount of respect for Sen. Clinton, and appreciate so much what she’s done for us," union president Andy Stern said Friday.
But he cited "excitement" among the union’s 1.9 million members as the major reason for their decision to endorse Obama – a nod that would supplant the state-by-state endorsements by their locals. He did not deny that recent vote results, most of which had favored Obama, had been a factor in the shift.
Thursday, Obama received another major labor endorsement likely to help in those delegate rich-states, when the 1.3-million member United Food and Commercial Workers Union decided to back his presidential bid.
Clinton recently received a major union nod, when the United Farm Workers decided last month to back her White House run.
–CNN Associate Political Editor Rebecca Sinderbrand