He rarely drops by diners or coffee shops with news cameras in tow, unlike most politicians. He hardly ever kisses babies, aides say, and does not mingle much at fund-raisers. To Mr. Sanders, the independent senator from Vermont, political schmoozing is a phony business, and anathema to his total focus on weighty issues.

After speaking last month at a house party on friendly liberal territory in Iowa City, Mr. Sanders ignored a dozen outstretched hands as he barreled down the driveway to a waiting car, pausing only when he came face to face with a woman who was in tears. She wanted to talk about mental health services for children like her daughter, whose needs, she said, required the family to pay for private school. Mr. Sanders listened for 30 seconds, embraced her with his right arm and walked away.

“He told me to ‘hang in there,’ nothing else,” the woman, Julie Casella, said afterward. “He seemed in a rush.”

In a feel-your-pain political era, many candidates would have bathed Ms. Casella in empathy (and made sure reporters were listening). But Mr. Sanders, who declined to be interviewed for this article, avoids making showy spectacles of other people’s pathos.

“He’s unslick in a really slick way,” said Rachel Joselson, a professor of music at the University of Iowa, who attended the house party. “That feels real and authentic to me.”

During a visit to William Penn University in Oskaloosa, Mr. Sanders took a question from Shelby Darland, 18, who described taking out a $20,000 loan at 8 percent interest to pay for community college. Mr. Sanders replied that student loan rates were exorbitant, and laid out his proposal to eliminate tuition at public colleges.

After leaving the stage, Mr. Sanders made a beeline for Ms. Darland.

“He told me that when I say stuff like that, I’m speaking to millions of Americans who aren’t ready to stand up yet and speak out,” Ms. Darland said.

But Mr. Sanders did not linger.

“I wanted to tell him how much I loved him,” she added, “but he had to go before I could say it.”