Retirements proved a tough hurdle for Republicans in House elections this year. Two years of divided government and a tough presidential cycle might make it worse in the Senate in the next election. Lamar Alexander, the three-term incumbent from Tennessee, decided to get the ball rolling by announcing his retirement in two years even before his last session of Congress begins early next month:

NEW: Sen. Lamar Alexander of Tennessee says he will not seek reelection in 2020. https://t.co/5Jc7jFN5rD pic.twitter.com/G7dynXprLP

— ABC News (@ABC) December 17, 2018

In one sense, Alexander’s doing the GOP a favor. The early retirement announcement gives Tennessee Republicans a long ramp-up time to find a strong nominee to run for the open seat two years from now. Had Alexander waited a few months, Democrats would have had a head start on that process by the time the need was made clear.

At 78 now and 80 in his re-elect year, perhaps the GOP might have expected a retirement from Alexander, but it’s coming at a tough time. Republicans will defend ten more seats than Democrats already in the 2020 election cycle, including an open seat in a special election in Arizona that looks less promising after this year’s midterm loss of Jeff Flake’s old seat. As I wrote after the midterms, there aren’t many bright spots in the next round, but a whole lotta worries going on:

Besides having to defend an appointee in Arizona after a stunning pickup by Sinema, they will also need to re-elect Cory Gardner in Colorado, which has also trended Democratic over the last few election cycles. Susan Collins will face a slightly Democratic electorate in Maine, which may not appreciate her full-throated support for Brett Kavanaugh. Joni Ernst will try for her first re-election in Iowa, where two Republican House incumbents lost to Democrats last week. Thom Tillis barely won his last re-election bid in North Carolina and can expect another stiff challenge. The only real bright spot for the GOP in 2020 will be in Alabama, where Doug Jones can expect his unlikely run as senator to end if Republicans nominate anyone besides Roy Moore.

By the way, 2022 looks just as bad for the GOP (as I noted in a previous version of the same column). In that cycle, Republicans will need to  defend Pat Toomey in Pennsylvania, where Bob Casey just cruised to re-election. Wisconsin’s Ron Johnson barely survived in 2014 even when the state was trending red and re-electing Scott Walker; he’ll have to win again after the state just re-elected incumbent Tammy Baldwin to her Senate seat and made fellow Democrat Tony Evers governor. Marco Rubio will face a tough fight in Florida, as will Richard Burr in North Carolina, Rob Portman in Ohio, and perhaps even Chuck Grassley in Iowa. The GOP’s only hopes of pickups will come in New Hampshire and Nevada, both of which went for Hillary Clinton in 2016.

Who else will retire over the next couple of years? Pat Roberts and Susan Collins might consider it; Roberts is older than Alexander and had a tough re-election bid four years ago. Collins is quite a bit younger than both but may be tiring of being a whipping post for ideologues on both sides. There appear to be few if any Democratic retirement candidates in Class II, and only Patrick Leahy as a potential retirement from Class III.

If a few more GOP seats open up with retirements, it will make it very difficult to get enough resources to hold those seats in states where Democrats have recently won statewide elections. It got just a little tougher to hold the Senate majority in 2020.