Given that control of the House is teetering and the battle over the 51-49 Senate is just short of hand-to-hand-combat, it may seem odd to be thinking about the 2020 Senate elections. But there is good reason to look ahead.

The odds are pretty overwhelming that coming out of November, neither party is likely to have more than 53 seats. If the Democratic wave is both real and huge, they could theoretically hold onto all 26 of their own seats and pick up all three competitive Republican-held seats, beating Sen. Dean Heller in Nevada and capturing the open seats in Arizona and Tennessee. Even that unlikely scenario would only get Democrats to 52 seats; they would have to win all of those plus pull off an extremely long-shot upset, such as beating Sen. Ted Cruz in Texas or appointed Sen. Cindy Hyde-Smith in Mississippi, to get to 53.

Conversely, given midterm-election patterns and President Trump’s extremely weak job-approval numbers, Republicans picking up two seats to reach 53 would be quite an accomplishment, and a three-seat win would justify a parade down Constitution Avenue. Ironically, given that this class of Senate seats is the most lopsided in modern history, if there were a Democrat in the White House today we’d probably be speculating on Republican chances of getting to a 60-seat supermajority in November.

The asymmetric partisan Senate exposure this year is a product of Democrats having a terrific 2006, when President George W. Bush’s job-approval rating had been hammered down to 38 percent in the Gallup Poll by the controversial war in Iraq, costing Republicans six Senate seats. When this class of Senate seats was last up, in 2012, the GOP lost two more, leading to this election with 26 Democratic seats up versus only nine for Republicans. To put it another way: This year, 53 percent of all Democratic Senate seats are on the ballot, compared to just 18 percent of those held by Republicans.

The disproportionate Democratic Senate exposure in 2018 is almost the mirror opposite of what awaits in both 2020 and 2022 (though of course whichever party wins the presidential race in 2020 will have to deal with the midterm-election curse in 2022). In 2020, there are 21 Republican Senate seats up to just 11 for Democrats, not counting the Mississippi and Minnesota seats held by Hyde-Smith and Democratic Sen. Tina Smith, respectively (both seats, created by vacancies, are up this year and again in 2020). The numbers in 2022 are very similar: Twenty-two Republican seats are up to just 11 for Democrats.

As usual, open seats could be an important driver in 2020. While the incumbent-reelection rate for senators is not as high as it is in the House, having a lot of open seats is often a source of heartburn for party strategists. Putting aside the aforementioned Mississippi and Minnesota seats, of the 11 Democratic seats up in 2020, five are held by incumbents who will be at least 70 years old at the time of the election and thus worth watching for retirement signs: Sens. Dick Durbin of Illinois, who will be 75; Ed Markey of Massachusetts, 74; Jack Reed of Rhode Island, 70; Jeanne Shaheen of New Hampshire, 73; and Tom Udall of New Mexico, 72.

Of the 21 Republican seats up in 2020, seven are held by senators who will be 70 or older at the time of that election: Sen. David Perdue of Georgia will be 70; Sen. Michael Enzi of Wyoming, 76; Sen. Mitch McConnell of Kentucky, 78; and Sen. James Risch of Idaho, 77. Three will be octogenarians: Sen. Lamar Alexander of Tennessee will be 80, while Sen. Pat Roberts of Kansas will be 84 and Sen. James Inhofe of Oklahoma will be 85.

Interestingly, the only seat up in 2020 in a state that the other party won massively is Sen. Doug Jones’s in Alabama, where Trump won by 28 points in 2016. Other Democrats up in competitive but not nearly as difficult states are Sens. Gary Peters in Michigan and Shaheen in New Hampshire.

The GOP has just three of their 21 seats that are up in states that Trump either lost or won by 5 points or less: Sens. Susan Collins in Maine, where Clinton won by 3 points; Cory Gardner in Colorado, which Clinton carried by 5 points; and Thom Tillis in North Carolina, where Trump prevailed by 4 points. These presidential numbers are worth paying attention to as more people are voting along party lines. The Pew Research Center recently computed that as recently as the mid-1980s, more than half of senators represented states that went for the opposite party’s presidential candidate. That number has since plummeted, and in 2016 every Senate seat went the same way the presidential race did in the state. The key question now is whether that will happen again in 2020.

This story was originally published on nationaljournal.com on July 17, 2018