Sanders is fairly mild left in global terms, the Democrat nomination has only a tangential impact on the Republican one.
To us yes, if he were a British politician I expect he would be far too right-wing to be in Corbyn's shadow cabinet. However in America is is regarded as pretty far left, in fact he used the 'S' word, and for a lot of Americans Socialist equates to Communist (which equates to a pact with the Devil).
So if he did manage to get the Democrat nomination, I would predict an overwhelming win for a right-wing Republican president, possibly Trump.