Democrats who run populous states are considering redistricting, but some of them face a hurdle: Their parties don’t have control over drawing legislative maps. Instead, the power is in the hands of independent commissions, something often left-leaning pro-democracy advocates have supported to enact fair maps that reduce gerrymandering.

Lawmakers are now framing their ability to prevent new maps from passing during one special legislative session as a victory. But Republicans are poised to call another.