At 78, Sanders is the oldest candidate in the Democratic field. But he projects vigor belying his age on the trail, and has not been faced with questions regarding his stamina and mental acuity that have plagued former Vice President Joe Biden, who is two years younger.

News of Sanders' procedure came as Advertising Analytics, an ad tracking firm, confirmed to POLITICO that his campaign had canceled its $1.3 million ad buy in Iowa on broadcast and cable television, which was announced Monday and set to begin Thursday. Mike Casca, a spokesman for Sanders, described the cancellation as "just a postponement" but did not elaborate.

Ethan Weiss, a cardiologist at the University of California, San Francisco, said the stent procedure Sanders underwent typically is “not anything I would get too excited about,” calling it "mostly a nuisance."

Weiss, who did not diagnose Sanders, added, "Unless I’m missing something dramatic, he should be in the hospital one or two days, and he should be fine.”

Late Wednesday afternoon, Sanders tweeted that he is "feeling good" and used the incident to plug his single-payer health care plan.

"Thanks for all the well wishes. I'm feeling good. I'm fortunate to have good health care and great doctors and nurses helping me to recover," he wrote. "None of us know when a medical emergency might affect us. And no one should fear going bankrupt if it occurs. Medicare for All!"