Rep. John TierneyJohn F. TierneyThe Memo: Trump back to relying on his instincts Two polls show tightening race for Tierney's seat in Mass. DCCC names Moulton as a top candidate MORE (D-Mass.) became the fourth House incumbent to lose his primary on Tuesday night, but challenger Seth MoultonSeth Wilbur MoultonMore Dems offer to hire McCabe McCabe is ‘considering all options’ after firing Democrats have shot at flipping the House if they can stay in the center MORE's victory could actually help Democrats hold onto the seat this fall. 

With 18 percent of precincts reporting, Moulton was leading Tierney 49 percent to 42 percent, and The Associated Press hadn't yet called the race, but just before 9:30 p.m., the 18-year congressional veteran conceded the race. 

Tierney's loss makes him the first House Democrat to lose a primary this year, but he's the fourth incumbent overall, joining Republican Reps. Kerry Bentivolio (Mich.), Ralph Hall (Texas) and House Majority Leader Eric Cantor (Va.). 

Tierney had survived a tough race in 2012 with ethics questions swirling around his candidacy, after his wife's brothers were indicted in an illegal gambling ring. Former lieutenant governor nominee Richard Tisei, an openly gay centrist Republican, gave Tierney a close challenge and has already been running a vigorous campaign, angling for a rematch with the incumbent. 

But now Tisei and national Republicans won't get that chance. With Moulton, the winning the Democratic-leaning seat is likely a harder task for the GOP, since they can't use Tierney's family baggage or his decades in Washington as a mark against him. 

Still, Republicans indicated Tuesday night their plan for the general election is to continue running their race against Tierney by making Moulton a stand-in for the longtime congressman.

"Seth himself has said repeatedly, in Congress, he would vote the same as John Tierney. That means Seth Moulton will rubberstamp the failed direction Washington is taking our country," said Tisei in a statement following Moulton's win.

He'll also look to frame himself as the only candidate with a proven record of getting things done for Massachusetts, during his time as Massachusetts Senate minority leader.

Moulton, a Marine Corps veteran who served four tours of duty in Iraq, was endorsed by retired Army Gen. Stanley McChrystal, which local Democrats said gave his campaign a big boost. Many observed that in the race's closing days, the momentum was with Moulton to pull off the upset over Tierney, and three polls in a row showed Tierney holding a razor-thin lead over the challenger.

Democrats rallied behind Moulton on Tuesday night, with Democratic Congressional Campaign Committee Chairman Steve Israel (D-N.Y.) congratulating him on his win in a statement.

Though the momentum was there, Moulton's quick win still caught the candidate off-guard. After he received the voicemail from Tierney conceding, Moulton, watching the returns at his apartment, scrambled to get dressed and head to his victory party.

In his victory speech, Moulton framed the race as a contrast between a stale politician and a fresh face, and argued his nomination is proof voters in the district want to keep it in Democratic hands.

"We won’t get fresh thinking and new leadership by sending someone to Washington who was first elected to office when I was just 6 years old," Moulton told supporters.

— This piece was updated at 11:30 p.m.