One need not prove fraud to substantiate the claim that it was stolen. It could have been stolen, for example, if laws were bent or broken to dramatically increase the number of mail-in absentee ballots, while at the same time decreasing the scrutiny given to those ballots as required by law.
In fact, this is exactly what was alleged in many court cases. These cases were not allowed to move forward, however, because of procedural issues, not the merits of the case (standing, mootness, and latches).
