Martin O’Malley, the former Maryland governor and Baltimore mayor who for the last year has served as commissioner of the Social Security Administration, on Monday became the first announced candidate for chair of the Democratic National Committee.

The entrance of Mr. O’Malley, 61, kicks off a race that is expected to have several candidates running to lead a party in turmoil after this year’s elections. Without control of the White House or either chamber of Congress in 2025, Democrats are in a defensive crouch, aiming to protect themselves and their constituents from a second Trump administration bent on right-wing policymaking and vengeance.

While the D.N.C. chair is considered a position of national prominence, the contest to succeed Jaime Harrison in that role is a highly insular version of a student council election. Only the 447 committee members get a vote in the election, which is expected to take place in early 2025, and relationships often matter more than anything else. Mr. O’Malley said that he was entering the race with three endorsements from D.N.C. members and had been in conversations with others in recent days. He resigned from his Social Security Administration post on Monday, effective Nov. 29.