Many of the rank and file believe that, and even they, on some level, know it's not true. Witness some of the insurrectionists currently being tried and sentenced, or the one who testified at the January 6 hearing. They got caught up in the moment, and believed their "president" needed their support--which in itself makes no sense, because, he is the commander in chief of a huge military and also has an entire administration at his disposal--why would he need scruffy militiamen? When they snap out of the haze of cult-followership and recognize they were under his "spell" or whatever, they realize the election was not stolen. And the donor class and Rethuglican representatives also know the truth. Cynically, self-servingly, they serve The Big Lie to bring themselves power, influence, and dollars.