(CNN) Georgia Republican Sen. Johnny Isakson's announcement Wednesday that he will resign from the chamber at the end of the year is just the sort of break Democrats hoping to retake the majority next November badly needed.

Here's why: Isakson wasn't up for reelection again until 2022. And had he run again, he would have been tough to beat given his long service to the state. But now, his seat will be on the ballot in 2020, not 2022. And whoever Gov. Brian Kemp (R) appoints to fill the immediate vacancy will have -- at best -- a year to convince voters that he or she deserves to serve out the final two years remaining on Isakson's term. (Also worth noting: The electoral record of appointed senators is not so good .)

None of that means, of course, that Democrats winning Isakson's seat is a done deal -- or anything close. After all, Georgia is a leaning Republican state that President Donald Trump carried by 5 points in 2016.

But in a way, that misses the point, which is this: Democrats desperately need to expand the playing field to have any sort of margin for error in their quest to win back the Senate in 2020. The addition of one more seat -- and one in a state where Democrats have been making gains at the ballot box in recent elections -- is a major boon in that effort.

Let's do the math.

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