Emily Brieve, a Republican county commissioner in Michigan, voted for Donald J. Trump in 2020. Her campaign website highlighted her opposition to abortion rights. And until this year, she had never considered voting for a Democratic presidential candidate.

But to Ms. Brieve, 42, the people with whom Mr. Trump surrounds himself seem increasingly “extreme.” His running mate, Senator JD Vance of Ohio, is “divisive” and “robotic,” ripe for caricature on “Saturday Night Live.” And after Mr. Trump’s Supreme Court nominees helped overturn Roe v. Wade, she thought some state abortion restrictions went too far.

“I’m still not 100 percent sure how I’m planning on voting,” Ms. Brieve, of Caledonia, Mich., said in an interview. “I just know that I’m not supportive of Trump, and I won’t vote for Trump ever again.”

In a bitterly divided nation, relatively few Americans are genuinely torn between Mr. Trump and Vice President Kamala Harris. Ms. Brieve represents a different yet crucial kind of undecided voter: one who has ruled out Mr. Trump but is grappling with whether to support Ms. Harris, write in someone else or skip the top of the ticket entirely.

In recent elections, center-right voters who have recoiled at the direction of the Republican Party — particularly college-educated suburbanites — have played significant roles in Democratic victories, helping propel President Biden in 2020 and shaping key 2022 midterm contests.

Now, in the final stretch of this campaign, Democrats see opportunities to expand that universe of voters. The party is betting that since Mr. Trump was last on the ballot, he has disqualified himself with more Americans who detest his election denialism and conspiracy theories, as well as his party’s abortion bans.