From Roll Call:

The GOP’s path to the Senate majority includes a mix of open seats and targeted Democratic incumbents. The two most vulnerable seats are in South Dakota and West Virginia, where Democratic senators are retiring. Republicans also have opportunities in open seats in Iowa and, to a lesser degree, Michigan.

But even if they are victorious in those states, the GOP must defeat at least two incumbents to reach the net six seats needed for control.

Luckily for Republicans, Democrats make up the vast majority of endangered senators seeking re-election. The GOP has a lengthy catalog of states where it has an opportunity to win, though there is a wide gap betweenthe No. 1 and No. 10 most vulnerable senators — who are ordered by most likely to lose.

Roll Call lists the #4 most vulnerable senator as Kay Hagan.

4. Sen. Kay Hagan, D-N.C.
Hagan’s approval rating is underwater in a red-leaning state where Obama’s is too, and outside groups have already started spending in what is expected to be one of the most expensive races this cycle.

But state Speaker Thom Tillis, Hagan’s Republican opponent, is off to slow a start. After narrowly avoiding a runoff, he headed immediately into the legislative session, where he has been stuck for the past dozen weeks. He finished the second quarter with just $1.5 million in the bank; Hagan had almost six times that amount.