DES MOINES, Iowa—There is no candidate in the country for whom Sen. Bernie Sanders has expended more political capital this year than Pete D’Alessandro.

Mr. Sanders hosted a rally for Mr. D’Alessandro, a U.S. House candidate who ran the Vermont senator’s 2016 presidential campaign in Iowa. He twice emailed his supporters soliciting campaign funds for Mr. D’Alessandro and recorded his only TV ad of the year to urge support for him.

Yet...