Former secretary of state Hillary Clinton hugs Sen. Barbara Mikulski, center, surrounded by Sens. Debbie Stabenow, left, Barbara Boxer, Dianne Feinstein and Patty Murray on Nov. 30. (Melina Mara/The Washington Post)

The largest union representing federal employees endorsed Hillary Clinton for president on Thursday, calling her a “a powerful ally in the ongoing fight to raise wages, create good jobs, expand benefits, and preserve workplace rights."

The American Federation of Government Employees said it interviewed Clinton and rival Sen. Bernie Sanders (I-Vt.) before making the choice. A statement from the group did not mention the third Democrat in the race, former Maryland governor Martin O’Malley. No Republican candidates responded to the union’s questionnaire, the group said.

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The AFGE represents 670,000 federal government workers in 1,100 locals nationwide and has been very active in past get-out-the-vote efforts. The union endorsed then-Sen. Barack Obama over Clinton in the 2008 race but was critical of Obama when he initiated a three-year freeze on the basic pay rates for federal employees after becoming president.

Clinton now has the support of nearly 12 million union members from 18 national unions and labor alliances, the former secretary of state’s campaign said.

“AFGE members make America run — protecting our national security, keeping our families healthy through food inspections, and serving our military in civilian support roles,” Clinton said in a statement thanking the union for its support.

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Clinton had already secured the backing of the American Federation of State, County and Municipal Employees, another powerful government workers’ union, and the two major teachers’ unions. Most of the other unions backing her are in the building trades and service industry. Sanders, despite his appeal to liberal Democrats, has secured far less union backing.

“Secretary Clinton shares AFGE’s vision for a strong and vibrant government workforce that has the necessary tools and support needed to deliver vital programs and services to the American public,” AFGE President J. David Cox Sr. said.

“The American people count on government employees to keep their streets safe, deliver their Social Security checks on time, and care for our nation’s heroes at veterans’ hospitals, and AFGE can count on Hillary Clinton to help us get it done.”

The union said it conducted a poll of members in addition to the in-person interviews in November. A majority of respondents said they planned to vote for a Democrat in the 2016 election, and respondents preferred Clinton by a 2-to-1 margin, the union said.