Besides, if Trump were to actually admit that the Russians interfered with the elections in 2016- to the extent that he believed our only recourse is to cancel the 2018 elections (and possibly 2020 as well), they cast aspersions on his own legitimacy (which is shaky at best). They'd also call into question the Republicans' legitimacy as well. There's not an argument in the world that they could muster where foreign interference in an election is sufficient reason to shut down our democracy. As for recent historical references, there *was* some "loose talk" back among some pundits and politicians during George W. Bush's "reign of error" that we could be "one terrorist attack away" from the imposition of some kind of martial law and Rudy Guiliani made a brief grab to stay in office longer, immediately following 9/11
States handle elections also, so how would Trump be able to order them shutdown? His order would (presumably) be shot down in court so fast it'd make his head spin.
What I have been most afraid of with Trump in terms of voting and election was his "voter fraud" commission, which I felt was going to be much more dangerous in terms of figuring out ways to further their already pretty effective voter suppression/disenfranchisement methods.