Ms. Egan, a graduate student at the University of Illinois Springfield, also wants the government to address the student debt crisis and the cost of higher education, even if she doesn’t support the sweeping loan forgiveness some Democrats have proposed. She recently graduated from Ohio University with $30,000 in debt despite academic scholarships, and was able to enroll in a master’s program only because of a tuition waiver.

Both she and Ms. Castro are loath to vote for Mr. Trump — who Ms. Egan argued used conservative ideas “as a smoke screen for a message and platform that’s based in anxiety, fear, suspicion and conspiracy” — but are deeply frustrated that their choice is him or a Democrat with whom they disagree on important policies.

Ms. Castro said that she would never vote for a Democrat who supported gun control — her one nonnegotiable issue — but that she wouldn’t vote for Mr. Trump either. She said she wished Representative Justin Amash, the Michigan libertarian who left the Republican Party last year, would run third-party. More likely, she will cast a write-in vote.

Much like young Democrats, many young Republicans say they are frustrated at the degree to which their views isolate them within the party.

“I’ve been called a RINO more times than I can count, which I don’t appreciate, because that’s their way of questioning my integrity and my principles,” Ms. Egan said. “There are plenty of other young Republicans, even if they’re not saying it, who feel alienated or ostracized because they’re being asked to support an ideology they didn’t sign up for.”

Their elders may say much of what is true now was also true decades ago. Boomers grew up with the threat of nuclear extinction and the view that previous generations had made a mess of everything from war and peace to sex. Many young Democrats were alienated from the party establishment and bitter when their favored candidates lost to more conventional ones. Rare is the political moment when the choices of the young trump those of older generations.

Still, in dozens of interviews, young voters on both sides of the aisle said that they felt their generation had been left to clean up after older ones, and that they resented what they saw as a choice by leaders in both parties not to prioritize the issues they cared about.