They've sent these to Senators because they already think Trump is making moves that seem eerily in line with moves Hitler made early in his career, e.g., declaring a war on the independent media, accusing state agencies of wrong doing or conspiracy.
The makers of these cards are trying to bluntly tell all US Senators that if they don't learn from history they will be doomed to repeat it and the consequences will affect us all, terribly.
The reputation of the President is controlled only by the President. And every word he's said recently has made his reputation worse, the guy just doesn't know how to control his brand...which is tragically ironic.