Democrats have been consoling themselves with the idea that the 2016 election is likely to look a lot more favorable to them than 2014 did. And that's true. Unless Republicans use their unprecedented sweep of state legislatures to change the rules and guarantee a victory for the GOP.
How would it work?
As National Review's Jim Geraghty explains, they could take advantage of the fact that the US Constitution grants state legislatures broad discretion in how they allocate their electoral votes to rig the system.
Right now, the way 48 states do it is they give all their electoral votes to whichever candidate won a plurality in the state. But Maine and Nebraska do it differently. They give two votes to the statewide winner, then one vote to the winner of each congressional district in the state. In practice, this makes very little difference. But as Geraghty points out it would make a huge difference if states like Ohio, Nevada, Florida, Michigan, Iowa, and Wisconsin did it. Democrats count on securing the electoral votes — all the electoral votes — of four or five of those states in order to win presidential elections.
Split them up, and it's basically impossible for Democrats to win a presidential election. Especially because district boundaries give Republicans an advantage in these states — it's often the case that Republicans win congressional districts well in excess of their share of the popular vote.
Would Republicans really do this?
Probably not. Pennsylvania Republican leaders strongly considered this in 2012 but backed down after a public outcry. Wisconsin Republicans went through a similar exercise. Which is to say this isn't a brand new idea, it's just an idea that so massively violates the established norms of American politics that nobody's gone through with it. Even Geraghty doesn't quite advocate it. He just outlines how it would work and ends with: "So . . . should Republicans pursue this course?"
But with Democrats now in control of state government in exactly zero red states, there's nothing stopping Republicans from going through with the plan. Indeed, there's nothing requiring Michigan (or any other state) from holding a presidential election at all. The state legislature could simply allocate its electoral votes to Mike Pence (or whomever) and tell angry liberals they should have thought about that when they decided not to turn out for the midterms.