What’s striking, say therapists, is how deep the fears go on both sides.

“About one-third of my patients talk about the election — and both the conservatives and the progressives are affected,” said David Rogers, a clinical psychologist in Hershey, Pa. “The campaign is arousing fears about the unknown. Both sides are aroused, because the results feel so consequential. It’s like when Lincoln was running against Douglas and Jefferson was up against Adams.”

Conservative voters are concerned about gun rights, and fear that Mrs. Clinton will appoint Supreme Court justices who will chip away at the Second Amendment. And in a world that feels increasingly unsafe, guns give some Americans a sense of security and control. On the other side, with Mr. Trump questioning the fairness of the election, and one Milwaukee sheriff calling for “pitchforks and torches,” some fear that the aftermath of the election could bring violence.

And the unusual nature of the election, with Mr. Trump pushing the limits of acceptable behavior in politics, is creating problems for people who struggle with mental health issues.

“Boundaries are an important part of mental health,” said Dr. Ducharme. “In order to feel safe, people need to have a sense that there are rules and norms. In this election our leaders appear to be out of control.”

Strong feelings about the candidates are causing not only rifts between Democrats and Republicans, but also among families and friends. Ms. Elias has patients who are avoiding friends and family members with whom they disagree.

“It’s like the Civil War,” said Ms. Elias. “I have never seen an election cause this level of conflict.”

“People are having a hard time seeing the perspectives of the opposite side,” said David Palmiter, a clinical psychologist in Clarks Summit, Pa.