New Hampshire Gov. Maggie Hassan won her state’s U.S. Senate seat, defeating Republican incumbent Kelly Ayotte in one of the most expensive races this year.

Ms. Hassan had 48.0% of the vote, compared with 47.9% for Ms. Ayotte—a difference of just 716 votes with all precincts reporting—when Ms. Ayotte conceded Wednesday.

“It has been a tremendous privilege to serve New Hampshire in the Senate,” Ms. Ayotte said in a statement. “This is a critical time for New Hampshire and our country, and now more than ever, we need to work together to address our challenges.”

Ms. Hassan, a well-liked governor, presented a potent threat to the popular Republican senator from the beginning of her campaign. An early ad depicting her upbeat family getting her son, who has cerebral palsy, off to school showed a warm personal side. Ms. Hassan, 58 years old, also touted her pragmatism, emphasizing themes like cutting wasteful spending, which made her sound almost like a Republican. Breaking with her party, Ms. Hassan was the only Democratic governor to call for a temporary ban on Syrian refugees.

Click for more from The Wall Street Journal.