Georgia (demographics) on our minds
Extrapolating the national statistics to Georgia's 9.7 million population (because I do not have state-specific data), we see:
6% of eligible, registered liberal voters did not vote (about 580,000 potential voters)
8% of eligible, unregistered liberal voters did not vote (about 776,000 potential voters)
If the Senate race in Georgia were not a runoff, Perdue would have beat Ossof by 88,000 votes
About 115,000 votes were drawn by the Libertarian candidate (Hazel)
I have not seen good data on how Libertarians break when forced to choose between a Democrat and Republican. I have seen reports that Libertarians are essentially Republicans in cool clothing. I have also seen another report suggesting that Libertarians break 2:1 in favor of Democrat candidates. Yet another report suggests that many would simply abstain.
Let's say one-third abstains, and we pose the worst-case scenario, that the remaining two-thirds all vote Republican. That would give Perdue another 77,000 votes. Thus, Democrats have to potentially find another 165,000 votes. Although that sounds daunting, it is clearly doable, with hard work starting now.
Getting 28% of nonvoting registered voters would suffice
Getting 21% of unregistered voters to register and vote would suffice
Assuming that getting someone to register and vote is three times harder than getting a registered person to vote, goals might look something like this:
Convince 124,000 more registered voters to vote
Convince 41,000 unregistered voters to register by 7 December and vote on 5 January