North Dakota: Heidi Heitkamp (D) vs. Kevin Cramer (R)

You might see North Dakota and think "why does North Dakota matter?" Well, this year, it does. Heidi Heitkamp is the incumbent Democrat in a state Trump won by a whopping 45 points. Heitkamp's challenger, Kevin Cramer, is currently the lone representative from the Roughrider state, so he has some notoriety within North Dakota. Republicans are fighting to keep a majority in the Senate, and flipping North Dakota is a big one for them. Right now, polls show Cramer with a slim 1.6-point lead.

Indiana: Joe Donnelly (D) vs. Mike Braun

Indiana is the first of two states where a Democrat named Joe is running in a vulnerable state for Democrats. This time it's in Mike Pence country. Republicans are going to fight their asses off to keep a majority in the Senate, and to do so, they may need to flip some Democrat seats. And Indiana could be one of them. Right now, Joe Donnelly, the incumbent Democrat, has an extremely slim lead in polls, and Democrats could be in danger if they treat the Midwest like they did in the 2016 election.

West Virginia: Joe Manchin (D) vs. Patrick Morrisey (R)

West Virginia is prime real estate for Republicans this year. In 2016, Trump won the state of West Virginia by a whopping 42 points. But incumbent Democrat Joe Manchin has an average lead of 9.3 percent in polls. Republican challenger Patrick Morrisey, the state's attorney general, will have a tough time taking down Manchin, who is running for his second full term in the Senate. Manchin benefits from having over 30 years experience in West Virginia politics, spending time as secretary of state and governor.

Arizona: Martha McSally (R) vs. Kyrsten Sinema (D)

On the night of Arizona's primary, Republicans sighed in relief when Martha McSally defeated Sheriff Joe and Dr. Kelli Ward in the primary. McSally is likely the GOP's best shot at retaining this seat vacated by Jeff Flake due to retirement. But the race is far from over. Despite Arizona being a conservative-leaning state, the poll average for this race shows Democratic challenger Kyrsten Sinema leading by an average of 2.5 points. If Democrats want to take a majority in the senate, Arizona will be crucial.

Ohio: Sherrod Brown (D) vs. Jim Renacci (R)

Ohio won by Donald Trump, but Democratic incumbent Sherrod Brown has held the seat since 2006 and has a healthy lead to retain the seat. RealClearPolitics has Brown up on Republican challenger Jim Renacci by an average of 15 points, which is nearly impossible but not unheard of to overcome. But with Brown being a moderate who at times with Trump, it'll be hard for Renacci to gain ground on Brown in Ohio, an important swing state.

Nevada: Dean Heller (R) vs. Jacky Rosen (D)

There are 10 Democrats running for re-election in states where Trump won in 2016. Dean Heller, the incumbent in Nevada, is the only Republican running in a state that Hillary won. If any state is going to flip in favor of the Democrats, it's going to be Nevada. The Silver State has voted Democrat in the past three presidential elections, and right now RealClearPolitics has Jacky Rosen over Heller by just under one point.

Missouri: Claire McCaskill (D) vs. Josh Hawley (R)

Missouri has become the top target for outside spending, and rightfully so. Democrat-aligned groups are spending money, close to $13.5 million, to keep the seat because Republicans know this is one of the most-contested seats in the entire country. McCaskill is seeking a third-term as senator from Missouri, but state attorney general Josh Hawley is seeking to thwart her efforts. McCaskill won re-election in 2012 quite comfortably despite Missouri voting Republican in each of the last five presidential elections. Republicans know that if they want to maintain a majority, Missouri will be key in doing so. Polls right now have Hawley with a lead of less than one percent.

Tennessee: Marsha Blackburn (R) vs. Phil Bredesen (D)

Sen. Bob Corker decided not to run for re-election, so this is a race for an open seat. Marsha Blackburn is the Republican nominee and the representative from Tennessee's seventh district. Phil Bredesen is a former governor of Tennessee, and he is the Democratic nominee. RealClearPolitics has this race in a statistical tie, and Democrats are keying in on this seat in order to steal the majority away from the GOP.

Florida: Bill Nelson (D) vs. Rick Scott (R)

Florida is arguably the most-competitive swing state in the country, so seeing that this race is essentially in a tie is no surprise. Rick Scott, the current governor for Florida, is likely the GOP's best chance to flip this important seat. This race also occurs on the same ballot as a contentious governor's race between Rep. Ron DeSantis and Tallahassee mayor Andrew Gillum. This seat could be the deciding factor if the Democrats take the Senate or if Republicans retain control.

Texas: Ted Cruz (R) vs. Beto O'Rourke (D)

Ted Cruz, one of the most-hated men in the Senate, may end up being a one-term senator because of one man: Beto O'Rourke, the representative from Texas's 16th district. O'Rourke has taken pride in visiting all of Texas's 254 counties and the fact that he has not taken any PAC or special-interest moneyany PAC or special-interest money. Texas has long been a solid-red states and one of the most important for the GOP in presidential elections. But O'Rourke trails Cruz by fewer than 5 points in most polls, according to RealClearPolitics.