The showdown between the two will not be held until January, possibly throwing control of the upper chamber into question until then.
The non-partisan Cook Political Report’s final prognostication put the race in the toss-up column, with RealClearPolitics’ average of several polls giving the Atlanta pastor a commanding 15.7 percentage point lead in its final tabulation.
Ms Loeffler was appointed to the seat last year by Republican Governor Brian Kemp following the resignation of Johnny Isakson.
The special election originally also saw a challenge from fellow Republican Doug Collins, a Georgia congressman. But on Tuesday night he tweeted that he was standing aside, opening up a run-off in January. He tweeted: “I just called @kloeffler and congratulated her on making the runoff. She has my support and endorsement. I look forward to all Republicans coming together. Raphael Warnock would be a disaster for Georgia and America.”
Democrats need to pick up four seats, or three plus the presidency, meaning a Vice President Kamala Harris could break any ties in their favor.
Ms Loeffler has been a close Donald Trump ally and an ultra-consistent Republican vote.
Meanwhile, the regular Senate race in Georgia is seeing a challenge from Democrat Jon Ossoff to Republican incumbent David Perdue.