At a news conference in Concord, Lamontagne said he would "humbly accept the verdict" of the voters and said he would not seek a recount, the Manchester Union-Leader reported.
Earlier, the secretary of state's office certified that Ayotte had won the election by 1,667 votes, the Concord Monitor reported.
Ayotte will face Democratic U.S. Rep. Paul Hodes in the November general election to replace retiring Sen. Judd Gregg, a Republican.
Early tabulations after polls closed Tuesday had showed Ayotte trailing Lamontagne.
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