Right now, Democrats are clear favorites in three seats Republicans currently hold: Arizona (Sen. Martha McSally), Colorado (Sen. Cory Gardner) and Maine (Sen. Susan Collins). They’re favored to defeat incumbents between about 2-in-3 times (65%) to three in four times (75%) in these states. All three are in states that were decided by 5 points or less in the 2016 presidential election, and where the national environment is helping the Democrats. The limited polling in Arizona and Maine also point to Democrats being ahead by a small margin.
To counter those three seats, Republicans are heavily favored in Alabama. Democratic Sen. Doug Jones won a shocking victory in a 2017 special Senate election. The polling and strong Republican tilt of the state indicate that Republicans should win this race about 6-in-7 in seven times (85%).
If Democrats are going to net gain three seats while losing in Alabama, their best shot to get that additional pickup is in North Carolina. This is another state that was determined by less than 5 points in the 2016 presidential election, and where Republican Sen. Thom Tillis has actually been running slightly behind Democratic challenger Cal Cunningham in an average of polling. Cunningham wins a little bit north of half the time (55%), though it’s best to regard this one as a tossup.