In 2014, Tim Groseclose, a professor of economics at George Mason University, looked over the 2010 and 2012 elections for the House and Senate and found that no candidate won if his district or state had gone for the other party’s presidential candidate by 13 points or more. The results held in 2014 and 2016 as well.
If his “rule of 13” holds up, then Republicans should do well in the Senate races–holding Tennessee and picking up Indiana, Missouri, Montana, North Dakota, and West Virginia. Even if they lose Arizona and Nevada, they would net three new votes. They have a chance to gain a seat in Florida, too.
In Montana and West Virginia, though, it would take an upset for the rule to hold up.