Georgia’s runoffs, the vestige of segregationist efforts to dilute Black voting power, will determine control of the Senate in races to be decided on Jan. 5. In the past, such contests have heavily favored Republicans because of a drop-off among Democratic voters, particularly African-Americans, after the general election.

But those intimately involved in the two previous Senate showdowns say what happened before is not necessarily predictive of the future. Demographic and cultural change has led to rapid shifts in the state, and Democrats have made concerted efforts to energize and turn out their voters, work that paved the way for President-elect Joseph R. Biden Jr.’s strong showing in the state.

“Both times before, Republicans really turned out and the Democrats didn’t,” said Saxby Chambliss, the former Republican senator of Georgia who won a second term in a 2008 runoff weeks after Barack Obama won the presidency. “This time around, I’m not so sure that is going to be the case. I have told my Republican colleagues that Democrats are fired up going into the race, and with Biden winning Georgia, I assume that gives them momentum.”