Though Democrats are poised to make significant gains in the House next month, thanks to a Senate map that's put them on the defensive in disproportionately red states, Republicans are still odds on favorites to maintain control of the upper chamber. Should they come up short this time, it may be natural to look toward 2020, when they hope an unpopular incumbent President Trump could weigh down Republicans, and bring Democrats unified control of Washington. While this is of course possible, it's worth noting that the 2020 Senate map is not particularly easy for Democrats, either.

On paper, the next round of Senate elections promises to be much easier than 2018. While Democrats have to defend 26 seats in 2018 compared to just 9 for Republicans, in 2020, it's Republicans who have to defend 22 seats compared to just 12 for Democrats. The difference is that this year, Democrats have to defend a number of Senate seats in hostile territory, such as North Dakota, Indiana, and Missouri, whereas in 2020, Republicans are on relatively safer ground.

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Specifically, of the 22 seats Republicans will have to defend in 2020, Trump carried 15 of them by double digits in 2016. Just two — Sens. Cory Gardner in Colorado and Susan Collins in Maine — are in states that Trump lost. Three more are in states that Trump won by within about five points: Arizona, Georgia, and North Carolina. One might want to add Sen. Joni Ernst as potentially vulnerable in Iowa, which has proven a swing state in recent political history, even though Trump carried it by 9.4 points in 2016.

The most vulnerable incumbent in 2020 is actually a Democrat — Sen. Doug Jones, who beat the infamously bad candidate Roy Moore by less than two points in a special election, and now will have to defend the seat during a presidential election year in a state that Trump carried by 27.2 points. Given his long odds, it's almost as if Democrats start off 2020 down one seat from whatever they have going into the next Congress.

Democrats also have to defend a seat in Michigan, which Trump won in 2016, as well as Minnesota and New Hampshire, where he came close. Though one imagines those seats would be safe in any race in which a Democrat wins the White House.

To be sure, it's hard to game out much this far in advance, without having any information on retirements, resignations, primaries, let alone the broader political climate. But looking at the map in 2020 tells us a few important things.

After 2018, it isn't just going to be significant whether Republicans maintain the majority, but whether they add to the majority. They could, in theory, remain the majority if it's 50-50 with Vice President Mike Pence as the tie-breaking vote. That would give Democrats a decent chance of retaking the Senate in 2020 in a strong year. However, if Republicans get up to 53 seats, Democrats may have trouble gaining the majority even if they retake the presidency.

More significantly from a policy perspective, any Senate majority that a Democratic president elected in 2020 will come in with is going to be narrow and reliant on several Senators representing red states. While liberal activists are pumped up and expect to urge on the presidential field to adopt bold positions, once in office, they are not going to have anything approaching the 60-vote majority President Barack Obama had. Ideas such as nuking the filibuster to pass single-payer are likely to have trouble getting far beyond the dreams of liberals and Republican attack ad makers.