Former Boston mayoral candidate John Connolly will endorse state Representative Jay Livingstone Monday in a tightly contested state Senate race.

Connolly, a former Boston city councilor, picked Livingstone over his one-time chief of staff, Diana Hwang.

“I like Diana,’’ Connolly said in an interview Sunday. “She worked for me for a very brief time. But I’ve known Jay for a decade as a very committed progressive. And I know he’ll be great for Boston.’’

Cayce McCabe, a spokesman for Hwang, said it is not surprising that Connolly is backing Livingstone, given that Livingstone endorsed Connolly during the mayor’s race and Hwang backed Martin J. Walsh, who won the contest.

Livingstone, of Beacon Hill, and Hwang, of East Boston, are two in a large field of Democrats vying to replace former senator Anthony Petruccelli, an East Boston Democrat who stepped down to take a job as a lobbyist.

The district includes neighborhoods in Boston, Cambridge, Revere, and Winthrop.

The primary is April 12.

David Scharfenberg