Why you are assuming this? Last time, Hillary Clinton received almost 3 million votes more than Trump and still accepted defeat. Because Trump won thanks to the electoral college. Grass root democrats were unhappy, but as far as I know there was no rioting or vandalism. On the other hand, I am not sure whether Trump will accept the same, if he loses the election.
It was a very different situation back in 2016. The people were also not encouraged to riot and to support the call for defund the police hehehe.
What can Trump do if he is not rightfully the winner under the electoral college? He cannot legally stay in the white house. The guards will force him out because he is not the commander in chief anymore hehehe.
As Vishnu.Reang pointed out in one of the previous posts, this time the counting is going to be very complex. More than half of the votes will be from mailed out ballots. Unlike the previous elections, the winner this time won't be declared for many days after the elections. This can obviously result in allegations of fraud, similar to what happened during the 2008 United States Senate election in Minnesota.
That does not change on what might occur if Trump wins or Biden wins. Trump cannot be sitting in the white house if he is not the rightful winner, however, the Democrats will protest the results if they lose while Trump is sitting in the white house.
I reckon Trump can protest but he will not be the sitting president.