look at all the voter ID laws and so forth they have put in place over lo! so many years! The GOP has been willing to smite all kinds of voters off the books to strain at gnats, basically. There really isn't this voter fraud of which Trump speaks--in fact, the illusion that there ever was was really a big old GOP whoop-de-do. This is not how voting works. It doesn't matter what spokesghoul Rudy Giuliani, who decries the problems of voting in the inner cities whilst having been a Mayor of NYC elected twice (so suburban, NYC, what?) and thinks dead people are doing so much of the voting, says. He, like Trump, should know better (and Giuliani probably does--Trump, not so much).
All elections are run on the state level. There is no overriding national vote frigging rigging. The elections have bipartisan oversight, and state government comes in both flavors: Republican and Democrat. It's just appallingly untrue for Trump to speculate about, and his saying it seems like a balm for his poll-wounded ego more than anything else.