Democrats' fear: 2020 race killing chances to win Senate

• Democrats need four seats to win a majority — but very few Republican incumbents look beatable right now. Susan Collins, who sits atop the list, is fairly popular in Maine. 
• The other two most vulnerable Republicans are Sens. Cory Gardner of Colorado and Martha McSally of Arizona, who's being challenged by Mark Kelly, the retired astronaut turned gun-control advocate.
• But even if Democrats somehow took out those three, they'll still struggle to hold onto the seat of Sen. Doug Jones in deep-red Alabama.

Many Democrats wish these 2020 presidential candidates — and possibilities — would run for Senate instead: Beto O’Rourke in Texas, John Hickenlooper in Colorado and Steve Bullock in Montana.

• In Georgia, Stacey Abrams has said no to the Senate but is still entertaining a presidential run.

In the House, Democrats will have a hard time making big gains:

• House Republicans need to pick up at least 18 seats to win back control. 31 Democrats represent districts that President Trump carried in 2016; another 12 represent battleground districts that Mitt Romney carried in 2012. 
• But House Republicans say it'll be difficult to win back control. Trump's likely 2020 path is so similar to his 2016 map that Republicans can't see flipping a lot of districts.

Go deeper: These Senate seats are up for election in 2020