U.S. Rep Niki Tsongas, who has served in Congress since 2007, will not seek reelection in 2018, a senior aide confirmed to the News Service on Wednesday morning.

Tsongas, who is the widow of the late Sen. Paul Tsongas, won a special election when UMass President Marty Meehan resigned his seat to become the chancellor of UMass Lowell. The Lowell Democrat representing the Third Congressional district has been re-elected five times.

"It has been my heartfelt honor to serve the people of this district over the past ten years and I have been guided all along by an extraordinary role model in my late husband Paul," Tsongas said in a statement Wednesday morning. "I have learned in life that there is a time for endings and for new beginnings. After much thought, I have decided that this is one of those times. The time feels right most especially because of my desire to spend more time enjoying and celebrating my wonderful and growing family."

During her tenure in Congress, Tsongas has focused heavily on military and veterans issues, and pointed in her announcement to her efforts to address sexual assault in the military and support for victims.

"I'm especially proud of the role I have been able to play in challenging the ways in which women are treated in the military, understanding that if you change the culture of one of our country's rightly honored bedrock institutions, you can change a country," she said in her statement. "That's why I worked to address the multitude of ways that women in the military, committed to serving our country, have been marginalized."

Tsongas said she was originally motivated to run by a "lifelong commitment to public service" and knowing that women could not win Congressional seats and have a say in federal policy unless they ran for office.

"I'm proud that my election marked the first time in a quarter century that Massachusetts sent a woman to Congress," she said. "Since that door cracked open, the Commonwealth has elected another female member of Congress, our first female US Senator, and in my district, 50% of our state legislators are now women, paving the way for even more women from our state to serve in political office bringing their voices to all we value as a country."

Her departure will open a seat rooted in the fourth largest city in Massachusetts. When Tsongas initially ran for Congress, the 2007 Democratic primary field she topped included current state Sens. Eileen Donoghue and Jamie Eldridge, then-Rep. Barry Finegold and Rep. James Miceli.