Vice President Joseph R. Biden Jr. swore in Sen. William “Mo” Cowan, D-Mass., as the state’s interim senator until a new senator is chosen in a June 25 special election.

Cowan is filling the seat of former Sen. John Kerry, who was tapped by President Barack Obama to be secretary of State. Kerry, who assumed his new duties on Feb. 1, was in the chamber and escorted Cowan to Thursday’s swearing-in.

Cowan was picked last month by Massachusetts Gov. Deval Patrick for the placeholder role. He is a trusted ally of the Democratic governor and formerly served as his chief of staff and chief legal counsel.

In a brief interview after the swearing-in Patrick said he was “proud.”

“This is an exceptionally well-prepared and highly thought of person who’s going to do a terrific job for the people,” the governor said.

At Cowan’s mock swearing-in, held in the Old Senate Chamber, Biden welcomed Cowan to the Senate. “I hope you love it,” the former Delaware senator told him. “I’m sure you’ll do a great job. I miss it.”

Cowan is the second African-American senator in the chamber, alongside Republican Tim Scott of South Carolina. Scott was on the floor when Cowan was officially sworn in, and Patrick said he was encouraged by their brief conversation. Scott “said a very hopeful thing in talking about maybe the two of them together can begin to show what it’s like to build a bridge,” Patrick said. “It’s a very promising idea.”

Prior to joining the Patrick administration in 2009, Cowan practiced civil litigation as an associate, and later as a partner in the Boston office of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, and he served as a special assistant district attorney in the office of the Middlesex County district attorney.

Cowan is a graduate of Duke University and Northeastern University School of Law, and lives in Stoughton, Mass., with his wife, Stacy, and their two young sons.

A special primary will be held April 30. Democratic Reps. Stephen F. Lynch and Edward J. Markey are the only declared candidates so far.

At this point it does not appear that Republicans will aggressively contest the race.

Republican state Rep. Dan Winslow announced Tuesday that he is making the first official moves to run in the special election. In recent days, many other Republicans, including former Sen. Scott P. Brown, have taken their names out of contention.