Widespread voter fraud is a myth until it is proven to exist.
Is there some degree of voter fraud? Of course. Every system is beatable if you put your mind to it.
But for the President to claim that he lost the popular vote due to voter fraud is patently ridiculous. If you do believe in widespread voter fraud, then you must believe that the system, that is in place to prevent it, absolutely sucks. If so, then it only makes sense that the voter fraud goes both ways, unless you think only Democrats are dishonest fraudsters and Republicans are all aboveboard and honest. The idea that fraud swung the popular vote 3-5 million in 1 direction makes absolutely no sense.
And to claim that busloads of voters were driven across NH state lines to turn the state against Trump also is ridiculous unless he has actual proof. Just think about red tape that would go into making that happen. You would need to identify citizens of NH who were not going to vote. You would then need to track down individuals willing to pose as those voters, without the story getting out to a member of the opposing party or an honest civilian who would blow the whistle. You would then need to make fake IDs for all of the fake voters you are bussing in to NH. You would then need all people involved in the plot to not tell the authorities or tell a friend/family member who goes to the press/authorities. The claim is ridiculous.