New Hampshire's 1st District has changed hands -- from Shea-Porter to Republican Frank Guinta -- in every election since 2010. Shea-Porter has served four terms since 2007, but lost the office twice in that span.
It's one of the nation's true swing districts. Both Barack Obama in 2012 and Donald Trump in 2016 won it by less than 2 percentage points.
In a statement, Shea-Porter -- the first woman ever elected to Congress from New Hampshire -- said she "felt the tug of family" at a recent reunion and has "continued to feel it" since then.
Her retirement could have serious implications for Democrats' efforts to win the 24 seats they need to take control of the House. So far, 19 Republicans and 10 Democrats have announced they'll retire or run for other offices in 2018. But Shea-Porter is just the third Democrat -- joining Minnesota Rep. Tim Walz and Nevada Rep. Jacky Rosen -- to vacate a seat where Trump beat Hillary Clinton last year.
Already, two Republicans -- state Sen. Andy Sanborn and Eddie Edwards, a Navy veteran and former South Hampton police chief -- are in the race. National Republicans said they already saw the district as one of their top pickup opportunities in the 2018 midterm cycle.
"This was already a top pickup opportunity even before Rep. Shea-Porter's announcement, and we are confident we will turn this district red once again," said Matt Gorman, the communications director for the National Republican Congressional Committee.
A long list of Democrats could emerge as contenders for the seat, though Shea-Porter's retirement came as a surprise.
"There is no doubt that Democrats will hold this seat, and we look forward to competing against whomever Steve Bannon nominates," said Democratic Congressional Campaign Committee communications director Meredith Kelly, referring to the former Trump White House chief strategist, who is wading into GOP primaries across the country.
CORRECTION: This story has been updated to reflect that Shea-Porter is just the third Democrat to vacate a seat where Trump beat Hillary Clinton in 2016.