Senator Jeanne Shaheen of New Hampshire will not run for re-election in 2026, bringing an end to a long and singular political career and further complicating Democrats’ efforts to regain a majority in the Senate.

Her decision not to seek a fourth term will immediately set off a high-stakes race in a state whose voters are famously fickle. Last fall, New Hampshire voters supported former Vice President Kamala Harris for president and elected Democrats to Congress, but they also voted for a Republican governor and expanded Republican majorities in the state legislature.

“It was a difficult decision, made more difficult by the current environment in the country — by President Trump and what he’s doing right now,” Ms. Shaheen, 78, said in an interview with The New York Times. She specifically criticized the president’s focus on political retribution, his drastic cuts to the federal budget and his antagonism toward Ukraine as it defends itself from Russia’s invasion.

Ms. Shaheen, the top Democrat on the Senate Foreign Relations Committee, was the first woman to be elected governor of New Hampshire and the first woman in the country to serve as both a governor and a U.S. senator. She noted in the interview that she will have served for 30 years in elected office and spent 50 years in politics.

“It’s important for New Hampshire and the country to have a new generation of leadership,” she said.