Story highlights New Hampshire's 1st District has changed hands in every election since 2010

It's one of the nation's true swing districts

Washington (CNN) New Hampshire Democratic Rep. Carol Shea-Porter announced Friday that she will retire at the end of her term, turning one of the nation's most competitive House seats into an open race.

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.

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