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.

Democratic U.S. Sen. Bob Casey of Pennsylvania and Republican challenger David McCormick meet for their second debate Tuesday evening, with control of the Senate on the line and tens of millions of dollars pouring into the race every week.

The battle for Pennsylvania’s 19 electoral votes may garner the most headlines nationwide, but the race for its U.S. Senate seat may determine whether Democrats will retain control of the upper chamber,