The district is considered a challenge for Democrats: Despite a creeping increase in registered Democratic voters in recent years, Republicans still outnumber Democrats by nearly 50,000. The last Democrat to hold the seat was Bill Owens, who won a 2009 special election and retired in 2014.

Ms. Stefanik, who graduated from Harvard in 2006, ran to fill Mr. Owens’s seat, after working as an aide to President George W. Bush and helping the 2012 Republican vice-presidential nominee, Representative Paul D. Ryan of Wisconsin, prepare for his debates.

Her sudden prominence comes less than a year after she clashed with Republican leaders over the party’s direction. Last December, after Republicans suffered big losses in the 2018 elections, Ms. Stefanik stepped down from her post as head of recruitment for the National Republican Congressional Committee, and circulated a letter arguing that Republicans’ lack of diversity — including having few female candidates — was hurting the party’s electoral chances.

“Neither our Republican caucus, nor our party as a whole, can afford further erosion among key demographics,” Ms. Stefanik wrote, in collaboration with three other members of Congress, adding that Republicans were “falling short in races that were otherwise winnable,” including in suburban districts.

Mr. Owens, a lawyer in Plattsburgh, N.Y., said that the district was changing somewhat in recent years but remained a rural, largely Republican, area — a place popular with hunters who have guns “and don’t misuse them.” He said Ms. Stefanik had initially been successful in presenting a moderate, bipartisan image, but felt that the Trump era had forced her to the right.

“She now appears very partisan,” Mr. Owens said. “And that’s not where she had been.”

As the sole Republican woman on the House Intelligence Committee, Ms. Stefanik’s questioning of Ms. Yovanovitch was both complimentary — with the congresswoman thanking the ambassador for her “tremendous public service” — and contentious, with Ms. Stefanik pressing the witness on her work on Ukrainian corruption during the Obama administration.

While Ms. Stefanik’s youth and gender make her an outlier among Republicans in the impeachment hearings, she rejected any suggestion that her enhanced role in the hearings was politically calculated.