Here are the races most likely to decide the balance of power in 2020.
Sen. Susan Collins (left)
The Maine Senate race is probably one of the most competitive out there. Current Republican Senator Susan Collins said back in January that she intends to run for reelection. Collins is more the moderate one of her GOP counterparts int he upper chamber. She will likely run a campaign that shows how she is "independent" from her party and how she has been able to work across the aisle. Maine is pretty Democratic, there's a Democratic governor, and all of the House seats in Maine are held by Democrats. She has shown that she can beat out tough opponents, but she might have to walk a pretty tight rope to win again.
Sen. Cory Gardner
Cory Gardner is one of the top targets for Democrats. He represents one of the states that were won by Hillary Clinton in 2016. The path for Gardner does not look easy. The state's governor, Jared Polis, a Democrat, won his race by double-digits. Gardner ran a great campaign in 2014 and beat out giant Mark Udall for his seat, but now it seems that path is dwindling. Colorado is bluer now, and Republicans have lost prime seats in the state.
Sen. Doug Jones
Trump won Alabama by 28 points in 2016. In 2017, Doug Jones narrowly beat out disgraced ex-judge Roy Moore in a highly publicized race. For Jones to win, the Republicans would have to nominate Moore to run, which probably won't happen. Jones was able to turn a ruby red state a tinge more purple with his win, but it looks like 2020 may be the end of the road for him. This race will most likely head back to the Republicans.
Sen. Martha McSally
Martha McSally lost her last race in 2018 to Kyrsten Sinema. After being appointed to fill John McCain's seat, she will have to run again in 2020, and again in 2022. Her most likely challenger is Mark Kelly, the former astronaut married to Gabbie Giffords. Giffords was the congresswoman who was brutally shot in an attack by a mad gunman. Kelly has remained consistent on his message that he is a moderate and is not too liberal. That worked for Sinema — will it work for Kelly?
5. North Carolina
Sen. Thom Tillis
North Carolina is the closest race other than those top four. Thom Tillis beat out Kay Hagan in 2014, in what many saw as a surprising upset. The state is a battleground, but it has been carried by the Republican presidential candidate in the last two cycles. Several polls in 2016 showed Hillary Clinton ahead, but of course, they were wrong, weren't they? Tillis is starting off well but could see a tough challenger.
Sen. Joni Ernst
This is one of the stated that is a long shot for Democrats. Joni Ernst is a no-bull farmer from the Hawkeye state. She stormed into the Senate in 2014 during the Republican wave that year. One of her campaign commercials made headlines after she talked about castrating hogs. Even in her victory speech, she said she would make Washington "squeal." Good one. Ernst is popular in Iowa, and it looks like she has a good shot at reelection.
Sen. David Perdue
Georgia is becoming a little bluer, but David Perdue has a solid red base behind him. The state was won by five points by Trump in 2016, less than the previous Republican presidential candidates. The odds of a Democrat winning her are pretty small. The Democrats would need someone like Stacey Abrams to run, in order for them to win. Bad news for them, Abrams said she isn't going to run for Senate in the Peach State this year. Things could change, but Georgia is most likely going red.