BIRMINGHAM, Ala. — VoteVets, the political action committee that backs liberal veterans running for office, has endorsed Pete Buttigieg for president, the first major organization to support him.

The endorsement brings Mr. Buttigieg, the mayor of South Bend, Ind., and a former Navy officer who served in Afghanistan, new defenders in a 2020 Democratic presidential contest that has seen better known candidates drop out as polling shows early-state voters coalescing around the four leading candidates.

Jon Soltz , the co-founder and chairman of VoteVets , said that other campaigns weren’t taking on Mr. Buttigieg directly but that they were using opposition research to try and “hold back his momentum.”