Runoff elections tend to be races of attrition. Turnout will most likely be lower, as voters are less accustomed to turning out for off-cycle elections. Candidates, then, must try to minimize attrition among their supporters, and the one with the least erosion is most likely to win.
Such was the case in Georgia on Dec. 6, 2022. Fewer people voted for either candidate in the runoff: Sen. Raphael Warnock, the Democratic incumbent, saw the number of people who turned out to vote for him drop by about 131,000 from the November vote; Republican Herschel Walker lost closer to 200,000 voters. This would explain how Warnock was able to grow his lead in the runoff.
