Whether Republicans do make gains will depend in no small part on these 20 races. …

The bad news? A majority of these twenty races constitute what is effectively an exposed Republican seat.

The good news? First, Republicans hold a 3 seat advantage in races that fall outside these 20 most competitive races; by my count, 9 seats are primed to be flipped by Republicans, compared to 6 seats for the Democrats. I’m comfortable saying this because in 2010, when races ended up outside the 57%-43% band, the seats very rarely changed hands. Republicans, then, start off with a roughly 3 seat advantage before we get to these 20 races.