Democrat Katherine Clark was sworn in Thursday as the state’s newest member of Congress and the fifth woman ever elected to the U.S. House from Massachusetts.

Clark took her oath of office on the House floor in Washington, joined by U.S. Rep. Richard Neal, U.S. Sen. Edward Markey and other members of the state’s all-Democratic congressional delegation.

Clark easily defeated three opponents in Tuesday’s special election. She resigned from her Melrose state Senate seat Thursday. An April 1 date has been set for a special election to fill that seat.

Clark will represent the state’s 5th Congressional District, which stretches from the coast to communities north and west of Boston, including Waltham, Framingham and Medford.

The seat opened this year when Markey won a special election to fill what had been Secretary of State John Kerry’s Senate seat.

In brief remarks after the swearing-in, Clark thanked her family and vowed to work for the needs of all families in the district and across Massachusetts looking for “a fair shot at the American Dream.”

“Our families want to be able to find a good job, send their children to great schools, save for college, and count on a secure retirement,” she said. “They want to know that the issues they talk about around their kitchen tables will be the same issues on the table here in Congress.”

She also talked about what she described as the diversity and strengths of the district, which she said is home to teachers, small-business owners, CEOs, machinists and “respected universities and innovative companies.”

“We are recent immigrants and descendants from early American settlers and yes, deep in the heart of Red Sox Nation, we even have a few Yankees fans,” she said.

Clark’s swearing-in marks the first time that three women are representing Massachusetts in Congress at the same time. Clark joins fellow Rep. Niki Tsongas from Lowell and U.S. Sen. Elizabeth Warren.

She also enters a Congress as the newest member of a minority party in a chamber where Democrats have few levers of power.

Clark ran as a liberal in one of the state’s most liberal districts. She supports abortion rights, gay marriage, tougher gun control measures, a higher minimum wage and pay equity for women. She also supports Social Security, early education, and President Obama’s health care law, saying it could pave the way to even broader health care coverage for Americans.

Clark, 50, a lawyer and former public interest attorney, will fill out the remainder of Markey’s term and face re-election next fall.

Massachusetts Secretary William Galvin said Thursday that the special election to fill Clark’s former seat representing the Fifth Middlesex State Senate District will take place April 1. The deadline for submitting nomination papers is Jan. 21.

That district consists of Malden, Melrose, Reading, Stoneham, Wakefield and four precincts in Winchester. Democratic state Rep. Jason Lewis from Winchester, has already announced his candidacy for the state Senate seat.

Clark lives in Melrose with her husband and their three children.

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