Senators also can set or defeat a president's legislative agenda.
Live election 2024 updates in the Philadelphia region and the focus on Pennsylvania
There are 34 seats up for grabs, and the fight for control of the chamber is coming down to a handful of races. Among those is Pennsylvania, where incumbent Democrat Bob Casey is locked in a bitter and expensive battle with Republican Dave McCormick.
"What we're probably going to see is that whoever wins the presidency is also their party's candidate for the Senate will also be the winner," said Professor Ben Dworkin of Rowan University Institute for Public Policy and Citizenship.
McCormick spoke at a Donald Trump rally in Reading Monday afternoon, hoping the top ticket will bring votes down the ballot.
"The choice for your Senate, President Trump needs a majority in the Senate. He needs a strong leader from Pennsylvania to get our country back on track. The choice between me and Bob Casey couldn't be more clear," McCormick said.
RELATED: Watch the full 6abc debate between Bob Casey, Dave McCormick
In the weeks leading up to the election, both candidates have highlighted their broad differences across issues like energy, abortion rights and the economy.
Bob Casey held a rally in Warrington alongside Senator Cory Booker.
"When you look at some of the numbers in the last couple of election cycles, the percent of the vote that a candidate gets in this county is very close to what they get statewide. So no pressure right," Casey said.
The outcome of this race is now dependent on Pennsylvanians.
"The bottom line is that nobody really knows. I mean yes, each side is confident, but it's a tie race, and it's all about get out the vote," said Dworkin.
6abc hosted a Senate debate where we heard from candidates on a wide array of issues. You can still watch the debate in its entirety in the video player below.
Watch the full 6abc debate between Bob Casey, Dave McCormick