Rep. Chris Van Hollen (D-Md.) at a rally in February. The Service Employees International Union said Thursday it will endorse Van Hollen and withdraw its longtime support for Rep. Donna F. Edwards (D-Md.) in the primary contest for the U.S. Senate. (Andrew Harnik/For The Washington Post)

The labor organization that helped launch the political career of Rep. Donna F. Edwards (D-Md.) is endorsing her primary rival in the race for the U.S. Senate.

The Service Employees International Union (SEIU) announced Thursday it would back Rep. Chris Van Hollen (D-Md.), withdrawing its support for Edwards, whom it had backed in previous elections. The congresswoman has credited the labor organization with having “put her over the top” in her bid for Congress in 2008, when she defeated an incumbent Democrat to ascend to the House.

With the endorsement, Edwards loses access to a significant funding source that had previously launched ads on her behalf and spent thousands of dollars in past elections for her campaign. She is also losing a robust volunteer network that helped popularize the former community activist’s name.

“We feel like she has turned her back toward labor,” said Ricarra Jones, political organizer for Local 1199, the largest SEIU chapter in Maryland, which has about 9,000 members. “We think that Van Hollen will do a much better job.”

The Edwards campaign responded by taking aim again at Van Hollen’s past support of free-trade agreements that Edwards has repeatedly opposed. Although Van Hollen criticized the most recent Trans-Pacific Partnership agreement, the Edwards campaign said his record on the issue has cost tens of thousands of jobs.

“That’s not a labor record that puts Maryland families first,” Edwards spokesman Benjamin Gerdes said. “Marylanders know Donna is the only candidate who will always have the courage to take on the Washington special interests because she will never forget where she came from.”

SEIU officials said chapters in the District and Maryland represent about 40,000 people, and members make endorsement choices. Their support gives Van Hollen a “strong ground game” with members manning phone banks, delivering mailers and knocking on doors for the candidate.

The union has fought at the forefront of some of the state’s biggest legislative battles, including the legalization of same-sex marriage, health-care issues and the Dream Act. Its endorsement is sought in many Democratic races across the state because of the amount of work it dedicates to campaigns.

“We are going to put boots on the ground in Baltimore City, Baltimore County and Prince George’s County,” where the union is strongest, Jones said.

Van Hollen and Edwards have split labor endorsements across the region in the increasingly competitive race. Teamsters in Baltimore and Washington are standing by Van Hollen. Meanwhile, in vote-rich Prince George’s, where Edwards lives, aerospace employees, machinists, electrical workers and the local Teamsters support her.

The largest state labor organizations, such as the American Federation of State, County and Municipal Employees, have been silent. One of the country’s most powerful, the AFL-CIO, elected to stay out of the race. The United Food and Commercial Workers Union’s Local 400 is expected to make endorsements next year.

But SEIU’s reversal, as one of the largest and most active unions in the region, plays into Edwards’s apparent diminishing poll strength and languid fundraising. A recent Baltimore Sun/University of Baltimore poll of likely primary voters showed that Edwards was 14 points behind Van Hollen, who had a 2-to-1 advantage in that metropolitan area after he started running TV ads there.

But an October poll by The Washington Post and the University of Maryland surveyed a random pool of Democratic-leaning voters and placed Edwards with a slight, six percentage point lead.

Nevertheless, the gap between the two campaigns’ contribution levels has widened with successive fundraising quarters. Van Hollen has about 10 times as much money in his campaign account as Edwards, according to the most recent filings.

The national women’s group Emily’s List, however, pledged this week to lift the Edwards campaign with $1 million in media advertisements. The congresswoman also earned the backing of national progressive groups early in the race.

For SEIU, the decision stems from discontent in Local 1199 over the sudden downsizing of Laurel Regional Hospital in Prince George’s that eliminated hundreds of jobs. The union has been lobbying state legislators to intervene and unsuccessfully filed a temporary restraining order against the hospital’s nonprofit operator, Dimensions Healthcare System.

Edwards’s support for the relocation of Washington Adventist Hospital to a site seven miles from Laurel disappointed activists, who said the approval of the project is partly to blame for the closure of the neighborhood medical facility. Although the lawmaker put out a news release lamenting the closure, activists said Edwards has done little else.

“As an organization we put in a lot of support, money and members’ time to support folks who will work for the best interests of working families,” Jones said. “We do not have permanent friends but we have permanent interests.”

In Van Hollen, the union sees a candidate who can effectively lobby for those interests including a $15 national minimum wage and paid sick leave at the federal level.

“When you are in the middle of a battle about Maryland’s progressive values, there is no one you want on your side more than SEIU,” Van Hollen said in a statement.