The upcoming Senate primary contests could change this outlook dramatically, but Larry J. Sabato's Crystal Ball has provided an early preview:

Note that we have long flipped Nebraska and North Dakota from Democratic control to Republican control; former Democratic Sen. Bob Kerrey’s return to Nebraska hasn’t moved us a bit. Note also, as we said above, we are assuming that Maine elects King, who in effect becomes an Independent Democrat akin to Connecticut’s Joe Lieberman or Vermont’s Bernie Sanders. Further, our analysis has Democrats holding seats that are actually or potentially competitive, such as Ohio, Michigan and Hawaii. Finally, we presume that Democrats don’t score surprising upsets in places like Arizona and Indiana.

Chart 1: Tilting the toss ups — ranking toss-up Senate races from most likely to go Republican (top, dark red tint) to most likely to go Democratic (bottom, dark blue tint)