More than half of the country now lives in states where the GOP controls the legislature.
The Washington Post says the GOP controls the legislatures in 30 states (31 if you count Nebraska, which is officially non-partisan). Republicans also control at least 31 governorships, meaning Republicans are in total control in 24 states. On the other hand, Democrats control just 6 states.
To boil it down, nearly 48% of the population live in states where Republicans control the government while just 16% live in Democratic controlled states.
That 24-6 split is actually significantly bigger than it was after 2010, when Republicans emerged from that wave election with complete control of 21 states, to Democrats' 11 — about a two-to-one advantage, versus today's four-to-one edge.
Back then, the GOP used those newfound majorities to dominate the redistricting process and push through things like new abortion restrictions, Voter ID laws, and many other things it had been unable to accomplish in most of the country.
No, state legislatures aren't the sexiest things in the world. But as a means for demonstrating a national wave, they're about as pure an indicator as you get. That's because they're the lowest-profile office (i.e. people vote the party more than anything) that is pretty uniform across the country. And as of today, the GOP is dominating in an unprecedented way.