Ciresi will be the first big-name Democrat to publicly back Horner, a former Republican who has tried to frame himself as the race’s only moderate candidate.
Ciresi has run unsuccessfully for the U.S. Senate twice in the last decade.
Horner faces DFLer Mark Dayton and Republican Tom Emmer. Horner has trailed his rivals in the latest polls, but has racked up a string of endorsements from moderate Republicans.
Ciresi and Horner are expected to attend a 2 p.m. Capitol news conference to announce the endorsement.