The neck-and-neck contest between Sen. Bob Casey (D-PA) and Republican challenger Dave McCormick is one of several that could be the Senate majority-maker.

Essentially, Cook’s team now considers the race too close to call, based on the results of recent internal polling commissioned by both campaigns and spending on political ads.

The Cook Political Report on Monday shifted the Pennsylvania Senate race into the “toss-up” category and moved the Nebraska Senate contest from likely to “lean Republican”