https://en.wikipedia.org/wiki/Gulag
And more importantly, it was a system that was both theoretically and practically founded on political violence. It didn't take it a couple of decades to get there and its the expressions of violence weren't limited to prison camps. Politically motivated forced collectivization, for instance, ran concurrently and ended up being exterminatory.
Illegal entry is a misdemeanor, thus criminal. QED.
Still, it seems like a rather desperate rhetorical move to describe even a reckless driver as "a criminal, by definition", much less a person fleeing political persecution and seeking refuge in another country — a right guaranteed to them by international law, even if the US is abrogating its obligations under that law. Technically, it's true that under US law they are guilty of a misdemeanor, just like a wino getting drunk in the park, but clearly neither of these is a central example of the class of people to which we normally refer when we say "a criminal"; it seems more like an argument for disregarding US law.
The "massive prison camp system full of political dissidents" I refer to was the GULAG system, which at times contained millions of people, slightly more even than the whole US prison system of today, of which a substantial fraction were suspected of nothing more than political disloyalty. But that didn't happen in ten or twenty years after the Russian Revolution. It took about a quarter century.