As the duo sing, “They’ve all come to look for America,” videos of thousands of supporters meeting Mr. Sanders, in different times and places, flow across the screen and divide into squares on a quilt that multiplies over and over until there are too many to see.

It was one of many ads that John Geer, a Vanderbilt University political scientist, and I showed to panels of people throughout the campaign. We ran a weekly experiment called SpotCheck in which we randomly assigned a representative sample of 1,000 people to see one of two campaign ads. We evaluated the ads’ persuasive effects and asked people to evaluate the ads on such criteria as whether the ad made them happy, hopeful, angry or worried.

By far, Mr. Sanders’s “America” was the ad from 2016 that made SpotCheck’s raters the happiest and the most hopeful. Nearly 80 percent of viewers said the ad made them at least a little bit happy and hopeful in the week it debuted — including over half of the Republicans who saw it.

We paired the ad with a spot from Hillary Clinton called “All the Good,” which also tested well. It featured the commanding voice of Morgan Freeman and a moving string soundtrack, yet only half the raters said this ad made them happy and hopeful.