The nation's partisan divide deepened at the state level, as Tuesday's elections put all but a few legislatures in the hands of a single party.

In 46 states, the same party now controls both chambers of the legislature, creating distinct divisions between red and blue states. The new alignment leaves only Iowa, New Hampshire and Kentucky with divided legislatures, the smallest number in almost 70 years. One state, Nebraska, has a nonpartisan, unicameral legislature.

The...