Cincinnati Fire Department officials estimated the crowd at Jet Machine at about 4,000.
Most were in the warehouse where Romney is to speak momentarily; others were in an overflow room next door.
Sen. Rob Portman, who was considered as Romney's running mate, is introducing Romney to the crowd now.
Portman made a pitch for early voting.
"They're open for business now,'' Portman said. "What do you say that after this, we all get in our cars and go down to Broadway (the location of county board of elections) and cast our ballots."