BOSTON (Reuters) - Representative Stephen Lynch plans to launch a run for the Senate on Thursday, challenging fellow Democratic congressman Edward Markey in a bid to succeed John Kerry, according to a source with knowledge of Lynch's plans.

A former ironworker who represents Boston and the surrounding area, Lynch has scheduled a series of appearances across the state, beginning in Springfield and Worcester - Massachusetts' third- and second-most-populous cities - and moving on to a rally at his own union hall.

The source declined to be identified ahead of Thursday's official announcement.

(Reporting by Scott Malone; Editing by Jim Loney)