This March 1, 2014 photo shows Seth Moulton, 35, speaking at a Democratic caucus in the library of Salem High School in Salem, Mass. (AP Photo, John Blanding, The Boston Globe)

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

Tierney's loss, which came as primary season ended across the country, was a blow to prominent Democratic establishment figures like Sen. Elizabeth Warren (D-Mass.) and House Minority Leader Nancy Pelosi (D-Calif.), who supported him. But Moulton's win may increase Democrats' chances of holding the 6th district, which lies north of Boston.

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Tierney, who was first elected to the House in 1996, delivered a brief concession speech to supporters thanking them for backing him. "This was just an amazing 18 years, and we appreciate it, and we appreciate you standing with us all the way," he told them.

Moulton, 35, is a first-time candidate who cast Tierney as an unproductive member of Congress and sought to tap into voter frustration with Washington. He ran an ad charging that Tierney missed "more votes than most other members of Congress" in his nearly two decades in the House.

Many Democratic strategists feel that Moulton's win will help the party against Republican nominee Richard Tisei, who was unopposed on the GOP side. Weighed down by a family gambling scandal, Tierney narrowly defeated Tisei in 2012 and faced the prospect of a difficult rematch against him.

Tierney joins former House majority leader Eric Cantor (R-Va.), Rep. Kerry Bentivolio (R-Mich.) and Rep. Ralph Hall (R-Tex.) as the only members of Congress to fall in primaries this year.

Tuesday marked the close of primary season, with voters also casting ballots in Delaware, New Hampshire, New York and Rhode Island. With eight weeks left until the midterm election, the most crucial period of the campaign has begun.

Former senator Scott Brown comfortably won the Republican nomination for U.S. Senate in New Hampshire on Tuesday and will continue his quest to unseat Sen. Jeanne Shaheen (D) this fall in one of the country's most closely watched races. The race is shaping up as a key battle in the fight for the Senate majority.

Veteran observers see Shaheen, a steady campaigner, as a favorite -- but hardly a lock -- against Brown, who has fought to overcome some early stumbles.

“She seems steady, but I don’t expect her to run away with this,” said University of New Hampshire political scientist Dante Scala. “I think Brown stays within shouting distance.”

Elsewhere on Tuesday, voters in New York and Rhode Island were settling key Democratic primaries.

In New York, Gov. Andrew Cuomo (D), who has been a frequent target of liberal complaints, easily defeated challenger Zephyr Teachout, a law professor who ran to his left. His running mate, former congresswoman Kathy Hochul, topped liberal law professor Tim Wu for the lieutenant governor's nod.

In Rhode Island, state Treasurer Gina Raimondo took a step closer to becoming the state's first woman governor, defeating Providence Mayor Angel Taveras and two other candidates in the Democratic primary. She will be favored to succeed outgoing Gov. Lincoln Chafee (D) in November.

In Massachusetts, state Attorney General Martha Coakley defeated two other contenders in the Democratic gubernatorial primary. 2010 nominee Charlie Baker secured the GOP nod once again.

In New Hampshire, Democrats control both U.S. House seats and the governor's mansion, leaving most of the notable primary action in the state to Republicans.

In the GOP primary for governor, former defense company chief executive Walt Havenstein, who was backed by New Jersey Gov. Chris Christie (R), defeated Andrew Hemingway, who headed up Newt Gingrich's 2012 presidential campaign in the Granite State. Havenstein will face popular Gov. Maggie Hassan, the nation's only Democratic woman governor. Hassan easily won her primary.

In the 1st district, former congressman Frank Guinta (R) advanced to a third consecutive match-up against vulnerable Rep. Carol Shea-Porter (D). Guinta defeated first-time candidate Daniel Innis, who is openly gay and was backed by a pair of well-funded super PACs.

In the 2nd district, state Rep. Marilinda Garcia, 31, won the GOP primary. Garcia will face Rep. Ann McLane Kuster, who like Shea-Porter, is looking at a tough reelection fight.

In Washington, Democratic leaders praised Tierney but looked forward to the Moulton-Tisei showdown.

“I congratulate Seth Moulton on his victory. The contrast could not be more clear between his candidacy and focus on putting Massachusetts middle class families first," said Democratic Congressional Campaign Committee Chairman Steve Israel (N.Y.) in a statement.

Tisei, who is openly gay, is viewed as a rising star in the GOP. Republicans expressed confidence in his chances.

"Unlike Seth Moulton, Richard is the kind of effective leader that the 6th District needs and will be able to add much-needed balance and a fresh perspective to the Massachusetts Congressional delegation," said National Republican Congressional Committee Chairman Greg Walden (Ore.) in a statement.