House Oversight and Government Reform Committee member Del. Eleanor Holmes Norton, D-DC, right, accompanied by Rep. John Tierney, D-Mass., left, speaks from the Democratic side of the Republican-controlled committee on Capitol Hill in Washington, in Washington, Friday, June 28, 2013, during a politically contentious session on whether to compel Internal Revenue Service official Lois Lerner to testify about the extra scrutiny the IRS gave to tea party and other conservative groups that applied for tax-exempt status. (AP Photo/J. Scott Applewhite) Rep. John Tierney (D-Mass.)  on Capitol Hill on June 28. (J. Scott Applewhite/AP)

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.