The heated Senate race in battleground Pennsylvania has tightened even further between Sen. Bob Casey (D-PA) and Republican challenger David McCormick as voters already start to cast their ballots. The neck-and-neck contest is one of several that could be the Senate majority-maker by determining whether Democrats hold on to the chamber with a razor-thin margin.

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”