Currently the GOP has a 53 to 47 majority but Democrats are trying to take control of the Senate for the first time in six years.
Historic amounts of fundraising has taken place, and Democrats are determined to unseat several Republicans in traditional red states, CBS News chief congressional correspondent Nancy Cordes reports.
In Georgia, Senator David Perdue is running neck and neck with 33-year-old Democrat Jon Ossoff.
"You shouldn't do everything that your handlers in Washington tell you to because you'll lose your soul along the way, senator," Ossoff told Perdue in a debate on Wednesday.
After that moment went viral, Perdue canceled their final debate, opting for a campaign rally with President Donald Trump instead.
In North Carolina, Republican Senator Thom Tillis, a supporter of the president, has trailed his Democratic challenger Cal Cunningham for months in what is now the most expensive Senate race in U.S. history. Cunningham admitted to an extramarital affair and it still didn't change the polls.
"People really need to understand what's a stake. A senator you can trust that made promises and kept them, or a candidate who's run on a foundation of trust and honor and he's been untruthful and dishonorable," Tillis said.
In Arizona, John McCain's former Senate seat could go blue. Republican Senator Martha McSally is under fire from former astronaut Mark Kelly over the GOP's response to coronavirus.
"Senator, you would understand this as a pilot, you guys did step one of the emergency procedure, and then you didn't do anything else. And that is a colossal failure," Kelly said at a debate last month.
Then there's Maine's Susan Collins, one of the Senate's most senior Republicans. She's being hammered for siding with the president in a state he lost by 3 points.
"Time and again making excuses for Trump's behavior," an ad for her challenger Sara Gideon says.
Democratic voters have poured money even into "reach" races like the one in South Carolina.
"They're killing us financially," Senator Lindsey Graham said on Fox News.
Now Graham's battle with Democrat Jaime Harrison is considered a tossup — in a state Graham won six years ago by 17 points.
"I wish the senator would take the urgency he has for the Supreme Court nomination to put that into actually doing a bill to address the needs of the folks in South Carolina," Harrison said in debate in October.
Democrats also are playing defense in a couple of races. They're worried about Alabama Senator Doug Jones keeping his seat, but in 13 competitive Senate races, 11 of those seats are currently held by Republicans.