President Joe Biden’s proposal this week to cap rent increases in larger apartment buildings across America has elevated an idea that’s long been embraced by progressive housing advocates in Colorado — but, in a split among Democrats, is opposed by Gov. Jared Polis. What’s more, at the local level, rent-stabilization policies are essentially illegal under Colorado law. Unveiled by Biden as he’s sought to ward off calls to step aside in the presidential race, the plan would cap annual rent increases at 5% for properties with more than 50 units.