U.S. Senator Jeanne Shaheen on Wednesday became the third Senate Democrat to say she would not seek reelection next year, further complicating her party's hopes of recapturing control of that chamber when President Donald Trump's term reaches its midpoint.

Ms. Shaheen’s retirement will set off a high-stakes fight for an open seat in a state where she has been a leading political figure for decades.

Shaheen's decision not to run again deals a significant blow to Democrats who were already facing a difficult path to reclaiming the Senate majority.