Tonight's results have already made clear that despite Democratic gains in the Sun Belt, they can't yet count on wins in states like Georgia, Arizona and Texas -- and are looking at more statewide losses in Florida.

Democrats are having a much better night, though, in the Upper Midwest. Democrat Gretchen Whitmer ran away with the Michigan governor's race, and suburban House seats in the state tipped in their direction. Their candidates are currently leading in the governor's races in Wisconsin, Minnesota and Iowa. And Republicans never even seriously contested statewide races in Pennsylvania.

There's much left to learn tonight, but these results so far would have major ramifications for 2020.

They suggest the Great Lakes states could be the most important battlegrounds -- which could shape which types of candidates Democratic presidential primary voters see as the most capable of defeating President Donald Trump, as well as the eventual nominee's strategy and choice as a running-mate.