Bernie Sanders’ 2016 run changed politics. His aides say it also changed Bernie Sanders.

On April 30, 2015, the day Sen. Bernie Sanders (I-VT) announced his first ever presidential run, more than three-quarters of the electorate had no idea who he was.

Sanders, of course, went on to give former Secretary of State Hillary Clinton a run for her money in the 2016 Democratic party primaries. His rallies attracted both enormous crowds and a healthy supply of young voters, for whom he articulated — in a thick Brooklyn accent — a vision of a government that he said...