Infrastructure was on the mind of nine Democratic presidential candidates and yet-to-announce contenders as they pitched their campaigns to Building Trades union members in Washington April 10.

The candidates were vying to be seen as organized labor’s biggest advocate in an increasingly crowded 2020 presidential race. At stake are coveted union endorsements and the potential campaign donations that follow.

While unions are unlikely to take a stance on candidates in the near future, the conference was a chance for Building...