BOSTON (Reuters) - Democratic Rep. Stephen Lynch, a former ironworker, on Thursday officially launched a bid for U.S. Senate, seeking the seat being vacated by John Kerry who was confirmed this week as the new U.S. Secretary of State.

Lynch will face off against fellow House member Edward Markey in an April 30 primary ahead of a June 25 special election to choose a permanent successor to Kerry.

In an online video, Lynch's campaign gives a broad-brush biography of the former ironworker, who grew up in public housing and ran a local union before going into politics.

"He'll go to Washington to stand up, not fit in," an announcer says.

Lynch has represented Boston and its surroundings in the House since 2001.

No prominent Republicans have said if they will run for the seat, and observers wonder whether former Republican Senator Scott Brown will seek a return to Washington.

Brown stunned the liberal state's Democratic establishment in 2010 when he won a special election to fill the U.S. Senate seat left vacant with the death of Edward Kennedy.

Brown lost a reelection bid last year to Democrat Elizabeth Warren.

Massachusetts Governor Deval Patrick, a Democrat, on Wednesday named his former chief of staff, William Cowan, to hold the U.S. Senate seat until a successor is elected.

Cowan told reporters he viewed the appointment as temporary and had no plans to run in the special election.

(Reporting by Scott Malone; editing by Carol Bishopric)