Rep. John Tierney (D-Mass.), already considered vulnerable in 2014, will now face a primary challenge from Seth Moulton, a former Marine who served in Iraq.
The congressman has been damaged by a scandal in which his wife pleaded guilty to helping file false tax returns for her brother, who ran an illegal gambling business in Antigua.
Moulton considered running for the 6th district as an independent in 2012 but ultimately decided against a bid. Tierney was reelected to an eighth term after narrowly defeating Richard Tisei, an openly gay Republican considered one of the party's stronger candidates.
Tierney is on the Democratic Congressional Campaign Committee's "Frontline" list of endangered incumbents who will get extra help from the party protecting their seats.
A graduate of Harvard Business School and the Kennedy School of Government, Moulton is running a health-care company in Massachusetts. Last year, he was working in Texas on a high-speed rail line. He plans to formally announce his campaign Tuesday.