Democrats probably need a better electorate in the runoff than they got in November, because even though Biden narrowly won, Perdue ran a little under two points ahead of Ossoff in his election, and the combined Republican vote outpaced the combined Democratic vote in the jungle primary special election by about a point. In other words, if the two Senate elections held in November had been conventional contests with a single Democrat facing a single Republican with no runoff, Perdue would have been reelected and a Republican very well could have won the other race, too.
In the general election, about 80% of all votes were cast before Election Day. Joe Biden won this chunk of votes by about six points. Donald Trump won the Election Day votes by 23 points, which got him within a few tenths of a percentage point of winning, but he fell short. If Democrats have done better in the pre-Election Day vote this time, then Republicans either need to win the Election Day vote by more than Trump did, or have the Election Day electorate make up a bigger share of the total votes cast (and still vote heavily Republican).
Could this happen? Sure. It may be that some Republicans who voted before Election Day in the general election will switch to day-of voting this time, perhaps in response to President Trump’s endless and unfounded complaints about the integrity of the Georgia election — complaints he reiterated in an Election Eve Georgia rally last night. More broadly, the voting patterns and methods of a post-holiday runoff may be different than those of a general election.