As you may have heard, Democrats have a state legislative problem. A lot's been written about this, but not a lot's gone into state-by-state detail, let alone district-by-district detail.
I made this page to try to remedy that. If you care about state legislative elections, then which elections should you follow this year? Presumably, the most consequential state legislative elections are ones that might change control of the chamber. I doubt there are many Democratic chambers at risk of going Republican, precisely because Republicans already have so many: There are 23 Republican-controlled chambers in states that voted for Barack Obama in 2012 (out of 52 chambers--nearly half!) and just one Democratic-controlled chamber in a state that voted for Mitt Romney (the Kentucky House). Presumably, you should focus on Republican chambers at risk of going Democratic, and the best candidates are presumably those 23 GOP-controlled chambers in Obama states.
Below are the 12 State Senates controlled by Republicans in states that voted for Barack Obama, sorted by how Obama did in the median district or districts. Districts are colored by the incumbent or most recent party (opaquely if they're up this year) and sorted by Obama's percentage from largest to smallest, with a dotted line at 50%. Hover over a district to see more information on the right, or click if you're on mobile.
(Florida redrew its State Senate map, so I included both the old map and the new map, and for these purposes the new map doesn't have any incumbents or history.)
And here are the 11 State Houses controlled by Republicans in states that voted for Barack Obama.
I'm not going to go through every interesting state or district right now, but a few examples of what this view helps make clear:
"Gerrymandering", in the sense of obviously unfavorable maps, is indeed a problem for Democrats in about half of these chambers, mostly in Wisconsin, Virginia, Michigan, Ohio, Florida, and Pennsylvania. The New Hampshire Senate and Minnesota House aren't great for Democrats either, and I've been told the New York Senate map is unfavorable for Democrats because of the local strength of certain incumbents despite seemingly favorable Presidential numbers. Still, the six states of Wisconsin, Virginia, Michigan, Ohio, Florida, and Pennsylvania have the most obvious gerrymandering effects.
The most consequential single state legislative election is probably in Colorado, where control of the State Senate is almost certainly going to come down to CO-SD-19--Republicans control the chamber by one vote and it's the only seat up for election where Obama got between 49% and 54.7% in 2012. The Arvada-based CO-SD-19 (53.5% Obama), a bit northwest of Denver, will have Republican incumbent Laura Woods, described by John Frank of the Denver Post as "one of the most conservative members of the State Senate", facing Democratic challenger Rachel Zinzinger, who she barely beat in 2014. (Zinzinger was the incumbent in 2014, but appointed after the previous incumbent, Evie Hudak, resigned during a potental recall--an effort lead by Woods.) Those are seemingly favorable conditions for Democrats, but not necessarily a sure thing, especially since Woods is now the incumbent.
The closely-divided Nevada Senate will probably come down to NV-SD-5, NV-SD-6, and NV-SD-15. Democrats would seem to have plenty of pickup opportunities in the Maine Senate, but I don't really know how they lost it in the first place. Democrats also have a lot of ground to make up to win back the Iowa House, but also a lot of apparent opportunities--it'd be interesting to know how the parties and other groups are organizing around those elections.
An interesting case is the Pennsylvania Senate, where Democrats have a seeming pickup opportunity--the Chester-area PA-SD-9, 54.7% Obama in 2012, and vacant after the resignation of Republican incumbent Dominic Pileggi--despite a statewide map and election schedule that makes winning the chamber nearly impossible. Democrats would have to win districts that Mitt Romney carried by twenty points or more to win the Pennsylvania Senate, so how much will either party prioritize a single district? (I think PA-SD-9 has both a special election in April and a regular election in November, where Democrats might have more favorable turnout.)
Sources: The vote totals and incumbent names and parties are almost all from the amazing people at Daily Kos Elections, although the new Florida Senate numbers are from MCI Maps. Election scheduling information came from Wikipedia, Ballotpedia, and the Washington Secretary of State.