“It is shocking that the race is in play,” he said, citing Ms. Boucher’s one unassailable advantage: her record of constituent service.

“That is something he cannot claim,” he said. “That’s what often sustains an incumbent is that they have built up a reservoir of good will among the residents of the district.”

But, he added, “I think there’s very little question that she feels challenged in a very big way.”

At a recent opening of a Republican field office in Ridgefield, Ms. Boucher certainly seemed like a candidate feeling the competition. She characterized Mr. Haskell as a “privileged, wealthy kid from a wealthy family in Westport” who only had opportunities to work in the offices of Mr. Murphy and Mr. Himes because of his family’s political donations to Democrats, according to a recording of her remarks obtained by The New York Times. But F.E.C. filings show Mr. Haskell’s parents have never made political donations.

Ms. Boucher has also accused Mr. Haskell’s campaign of stealing lawn signs and conducting a push poll. Mr. Haskell has denied both accusations, pointing to his limited campaign resources and campaign finance records that show his campaign has not spent any money on polling.

Ms. Boucher and her campaign dodged numerous requests for comment over the course of several days, asserting that the senator did not have the time to speak.

Last week, when The Times approached Ms. Boucher after her first candidate forum with Mr. Haskell, she said she had no time to talk because of another event she had to attend that evening. Ms. Boucher then lingered at the forum for at least another 15 minutes, chatting amiably with constituents and the other Republican elected officials who appeared at the forum.