Rep. John F. Tierney (D-Mass.) was defeated by Iraq war veteran Seth Moulton Tuesday, becoming just the fourth sitting member of Congress to fall in a primary this year.

Tierney told supporters Tuesday that he was conceding.

"This was just an amazing 18 years, and we appreciate it, and we appreciate you standing with us all the way," Tierney told them.

With 27 percent of the vote tallied, Moulton led Tierney 51 percent to 40 percent. Three other challengers lagged behind.

Moulton, who served in the Marines, will face Republican Richard Tisei in November. Tisei, who is openly gay, lost a close race to Tierney in 2012. He was unopposed for the Republican nomination.

Moulton's win comes amid a robust effort from Tierney's allies to boost him. Sen. Elizabeth Warren (D-Mass.), a liberal superstar, endorsed the congressman. He also received fundraising support from House Minority Leader Nancy Pelosi (D-Calif.).

Moulton ran an ad charging that that Tierney missed "more votes than most other members of Congress" in his nearly two decades in the House. The ad also sought to tap into voter fatigue with Washington.

Tierney parried with an ad accusing Mouton of being tied to conservative special interests.

Tierney was first elected to the House in 1996. He was hamstrung ahead of his challenging 2012 reelection campaign by a family gambling scandal that received widespread attention in his district, which lies north of Boston. He has long been viewed as one of the most vulnerable members running for reelection this year.

The only other congressional incumbents to lose so far this year have been former House majority leader Eric Cantor; Rep. Kerry Bentivolio (R-Mich.), who raised little cash; and Rep. Ralph Hall (R-Tex.), the oldest member of Congress.