State Senator Katherine M. Clark bested six Democratic rivals Tuesday, winning her party’s nomination in the race to succeed Edward J. Markey in the US House of Representatives and setting her on course to likely become the state’s newest member of Congress.

Clark, a Melrose lawyer captured 31.6 percent of the vote. Middlesex County Sheriff Peter J. Koutoujian and state Representative Carl M. Sciortino trailed with 22 percent and 16.1 percent, respectively.

As the Democratic nominee in a liberal district north and west of Boston — one that voted by more than 30 percentage points for President Barack Obama over Mitt Romney last November — Clark is now the strong favorite going into the December general election. She will face Frank J. Addivinola Jr., the Republican nominee, who won the Republican primary Tuesday night.

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Should Clark, 50, win on Dec. 10, she would become only the fifth woman in history to represent Massachusetts in the US House.

“I’m so unbelievably proud of the campaign we’ve run. Because I believe ours is a cause worth fighting for,” Clark was set to tell supporters who gathered at the Knights of Columbus hall in Melrose to celebrate her victory Tuesday night. “A cause that says Republican extremists have to stop attacking women’s rights and start solving the issues facing our families.”

Addivinola, a lawyer and businessman from Boston, which is not in the district, claimed 49.1 percent of the vote on the Republican side. Attempts to reach him Tuesday were unsuccessful.

Outside political analysts see Clark’s triumph as due in part to the fact that she jumped into race months before Markey won a June special election for Senate.

Clark filed a statement of candidacy with the Federal Election Commission — a necessary step to begin fundraising — in mid-February. And early in her effort, her campaign was already going door-to-door to identify voters who might favor her in the primary.

Campaigning across the mostly suburban district, which covers all or part of 24 towns and cities from Winthrop to Holliston, Clark focused on women’s issues and on knocking Congressional Republicans.

“The extremist Republicans in the House . . . are trying to take away a woman’s right to choose, they are trying to deny millions of Americans the right to affordable health insurance,” she said at a Democratic voter forum in September, a theme she hammered throughout her months on the campaign trail.

Clark, whose campaign raised almost a million dollars, aired three television ads in recent weeks. In each, she derided Republicans and underlined her support for equal pay for women in the workplace and female reproductive rights—hewing to well-worn Democratic talking points.

But beyond the standard mechanisms of a Congressional campaign, Clark, who is warm and engaging in person, also ran something of a under-the-radar kaffeeklatsch effort. The Congressional hopeful attended several dozen informal sessions at voters’ homes in the district, in what she called a “Kitchen Table Tour.”

Sometimes the sip-and-schmooze sessions were attended by only a few people, sometimes by a roomful. But in each, Clark had an opportunity to speak at length and answer voters’ questions. Campaign aides believe this effort has a multiplying effect. Voters who met Clark and liked her, shared their enthusiasm with friends and neighbors.

Clark was also boosted by outside spending by EMILY’s List, a well-funded national group that supports female candidates who favor abortion rights. An arm of that organization spent at least $145,000 on phone banks and mailings in recent weeks, according to filings with the Federal Election Commission.

She was also endorsed by Attorney General Martha Coakley who was set to introduce Clark at her victory party Tuesday night.

Born and raised in New Haven, Conn., Clark lives with her husband and their three school-age sons in Melrose.

A graduate of St. Lawrence University, where the majored in history, Clark also has degrees from Cornell Law School and Harvard’s Kennedy School of Government. A former member of the Melrose School Committee, she was first elected to the Legislature as a state representative in 2008, winning a state Senate seat in 2010.

Before becoming a lawmaker on Beacon Hill, Clark served as Chief of the Policy Division in Attorney General Martha Coakley’s office and as general counsel for the Massachusetts Office of Child Care Services.

Though she now moves into the general election season as the heavy favorite, Clark still must beat Addivinola before she cements a win, a challenge not lost in her victory speech Tuesday night.

“With your help, on December 10th, I’m going to go to Congress and do my part to bring your voice and your family’s concerns to the table in Washington,” she was set to say in her remarks.