“This election is most like 1980,” Peter D. Hart, a veteran Democratic pollster, said of the election in which Ronald Reagan’s large victory also delivered the Senate to Republicans. “If the presidential election narrows to less than 3 percent, then Democrats will need some good fortune to win control of the Senate. My advice is to watch the margin between Clinton and Trump.”

Unlike many other races around the country, the fight for the Senate seat in New Hampshire features two women whom voters know well in a state where most elected officials seem familiar with at least half the people they run into. Ms. Ayotte was the state’s attorney general before she was elected to the Senate, and Ms. Hassan is a product of the Democratic mentoring system under Senator Jeanne Shaheen, another former governor.

Both have run well-calibrated campaigns focused on the state’s opioid problem and fiscal future. Both have been accessible to voters, who expect it here, with six debates, 10 candidate forums and dozens of campaign stops.

Ms. Ayotte runs a five-kilometer race almost every Saturday in towns around the state, and favors truck stops and town dumps to meet voters. She will campaign for 24 straight hours beginning Monday morning. “It’s going to be fun,” she said, perhaps redefining notions of enjoyment.

Ms. Hassan has relied on the star power of her party, campaigning in recent weeks with Senator Bernie Sanders of Vermont, who won the state big in the Democratic primary; as well as Senator Elizabeth Warren of Massachusetts; Michelle Obama, the first lady; and Vice President Joseph R. Biden Jr. “I am just working hard every day,” Ms. Hassan said at a bakery last week.

Ms. Ayotte has struggled mightily with the candidacy of Mr. Trump, at first giving him lukewarm support. And when asked during a debate if she saw Mr. Trump as a role model for youths, she replied, “Absolutely.” She then put out a statement contradicting the remark, which became a bludgeon for Ms. Hassan. Ms. Ayotte finally dumped Mr. Trump, a dizzying ride that may have tainted her brand of independence. Ms. Hassan has had her own troubles with Mrs. Clinton.