Hillary won the popular vote, remember. Trump has only gotten less popular since then. The only reason why Trump is accusing Democrats of cheating this early is because he wants to discredit the results of the election *** in case he loses ***. Because if he wins then the election will be valid to him, regardless of all the mail-in voting which could, btw, be used to help him, too.
11 intelligence agencies found Russia interference. The Senate Intelligence Committee (led by Republicans) said Russia access voting machines in all 50 states. If you don't think Trump cheated to win the first time, he's definitely going to try now because the half the freaking world hates him this time.