With the Republican majority in the U.S. Senate currently standing at 53-47, Democrats need to net four seats in the 2020 election to take control — or three, if they also win the presidency and therefore the vice presidency, the tie-breaking vote in the chamber. Democrats’ opportunities to do so expanded on Wednesday, when Georgia Sen. Johnny Isakson announced that he would resign at the end of the year. Isakson, a Republican who has served in the Senate since 2005, was diagnosed with Parkinson’s disease in 2015 and cited his “mounting health challenges” as the reason for his decision.

What happens next is that Republican Gov. Brian Kemp will appoint a new senator to serve until the next regularly scheduled general election (in this case, November 2020), at which point a special election will be held to decide who serves the rest of Isakson’s term. Because Isakson was not scheduled to be up for reelection until 2022, the upshot for Democrats is that one extra Republican-held Senate seat will be on the ballot next year — and it comes in a state that could be competitive.

With FiveThirtyEight’s partisan lean of R+12, Georgia is still a red state. But the steady growth of the state’s nonwhite population and the defection of voters in well-educated suburbs (such as those around Atlanta) to Democrats in the Trump era have caused it to drift left. In the 2008 presidential election, Georgia was 12.5 points redder than the nation as a whole; in 2012, it was 11.8 points redder; in 2016, it was 7.3 points redder. It is reasonable to expect, then, that Georgia could be even closer to the tipping point in 2020. In other words, a good national cycle for Democrats — or a good Democratic candidate — could be enough to flip the seat blue (or at least come close).

Furthermore, Georgia’s other U.S. senator, Republican David Perdue, was already up for reelection next year. That’s a big deal because chances are that the same party will win both Senate seats. Why? Because when both of a state’s Senate seats are on the ballot at the same time, they almost always go the same way. The last time there was a split decision in one of these “double-barrel” Senate elections was 1966. So Democrats might try harder in Georgia in 2020 than they were already going to, as they could get two Senate seats for the price of one. That said, with Senate results increasingly determined by presidential partisanship, both seats may simply go the way of the presidential race — and right now, handicappers expect President Trump to carry Georgia in 2020.

So who might run? For Republicans, Kemp may try to appoint someone who would then turn around and run in the special election, in the hopes that it would confer an incumbency advantage. But incumbency doesn’t benefit appointed incumbents as much as those who are elected. (It might help clear the GOP primary field, though, which could be especially important in this election — more on that in a moment.) Kemp hasn’t said yet who he’ll appoint, but the Atlanta Journal-Constitution offers Lt. Gov. Geoff Duncan, Attorney General Chris Carr and U.S. Secretary of Agriculture Sonny Perdue as potential candidates or appointees.

For Democrats, the question is whether they can field a formidable opponent. The obvious candidate — former state House Minority Leader Stacey Abrams — has already removed herself from consideration. Abrams, who came within 1.4 percentage points of defeating Kemp in the 2018 governor’s race, quickly issued a statement saying she would not run. But in the leadup to the 2018 election, Abrams herself was untested in a statewide campaign, so it’s not impossible that some currently unknown Democrat could emerge.

There’s also one final twist to be aware of — the reason it would behoove each party to rally around a single candidate: Instead of a normal primary followed by a general election, all candidates, regardless of party, will run in a single “jungle primary” on Nov. 3, 2020. So if no candidate receives a majority, the top two finishers will advance to a runoff. This makes the race extra unpredictable, as any runoff would occur without the increased turnout of the presidential election influencing the results of the race.

Finally, it feels like we say this about some runoff somewhere every election cycle and it never comes true, but it is possible that control of the U.S. Senate could hinge on the result of Georgia’s special-election runoff. That means we might not know which party is in charge until the cycle is essentially over: The runoff would take place on Jan. 5, 2021 (!), which is two days after the new Senate is scheduled to be sworn in.

In order to net three or four Senate seats, Democrats will need to be competitive in a broad array of races. And while that field is relatively small thanks to the Senate’s small-state and Republican bias, which reserves the same number of seats for a sparsely populated state as for a crowded one, the placement of this seat on the 2020 map gives Democrats one more path to the majority than they had at the beginning of the week. Although they remain underdogs in the Peach State, that is still good news for the party.