It means he supported Cato the Younger. Cato's whole deal was a return to what he considered old morality (although he was also an adulterous hypocrite) and a defense of the Sullan constitution. Cato's more specific problem was a staunch opposition to anything that Caesar did--it was Cato that got the order sent to Caesar to disband and come to Rome for prosecution. The Catonians joined up with Pompey at Pharsalus and after their defeat there went to Africa, where they were destroyed along with the rest of the remaining Republicans (which at that point was basically the same thing as the Catonians) at Thapsus