Bill de Blasio has risen to the top of the polls assailing the Bloomberg administration, but if elected he could pursue even more aggressive policies than his predecessor on a crucial issue: creating densely packed new residential towers through land-use decisions.

Mr. de Blasio, the city's public advocate, would push for mandatory affordable housing and fewer tax breaks for developers. But he wouldn't differ from Mr. Bloomberg on a fundamental premise that building significant amounts of new housing is a top way to spur...