Cynthia Shaw worked at a polling place in the Detroit suburbs on Election Day and went to bed “still hopeful” that Vice President Kamala Harris could win, she said. By Wednesday morning, she was bereft, her head pounding.

“It feels so much more definitive this time,” Ms. Shaw, 65, said of Donald J. Trump’s victory.

After Mr. Trump won the presidency in 2016, many Americans who had opposed him became dedicated activists. They used Facebook to organize marches, joined protests against his policies and formed new organizations to recruit liberal candidates for office.

Now that Mr. Trump is president-elect once more, preparing to lead a still-divided country that voted more decisively in his favor this time, many of those same people are wondering if they can summon the strength to do it all — or even some of it — over again.

“So many of us are so exhausted,” said Ms. Shaw, a Democrat who has volunteered in every presidential election since 1992. “I don’t mean to be so bleak, but that’s how it feels today.”

In Arvada, Colo., Liz Folkestad, 43, allowed herself to stay in bed a little longer than usual on Wednesday morning, entertaining fantasies of escape.