Washington (CNN) With the 2018 midterm elections over, the stage is now set for 2020, a year that will be defined by a presidential election and another fight for control of Congress.

Democrats faced a more challenging Senate map in 2018 than Republicans, in part because they had more seats up for election. But in 2020, the GOP will have to defend more Senate seats than Democrats. As of now, there will be 22 Republican Senate seats on the line, while Democrats will only need to protect 12 seats.

Numbers alone don't tell the whole story, however. Many of the seats Republicans have to defend in 2020 are in solid red states like Wyoming, Arkansas and Oklahoma. Democrats, meanwhile, appear to have the most vulnerable seat on the map in Alabama, a state Trump won by double digits in 2016.

The Democrats who will have to defend seats in states Trump won are Sens. Doug Jones of Alabama and Gary Peters of Michigan. The Republicans up for re-election in states Hillary Clinton won in the 2016 election are Sens. Susan Collins of Maine and Cory Gardner of Colorado.

Another race likely to be competitive is the open Senate special election in Arizona. Republican Sen. Jon Kyl, who was appointed to the seat held by the late Sen. John McCain, has said he will not run in 2020 when a special election takes place.

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