Some recent polls show Loeffler with a slight edge over Collins in the contest, which features all candidates on the same ballot with no primary to filter nominees.
Democrats Matt Lieberman and Raphael Warnock are among the 20 candidates challenging Loeffler in the wild contest, which is likely to be settled in a January runoff between the two top finishers.
With Butler’s endorsement, only a handful of top Georgia Republicans remain neutral in the race. U.S. Sen. David Perdue, who is also facing a November election, has stayed on the sidelines. So has state Superintendent Richard Woods.