What they aren't saying when they jump on Trump's criticism of Judge Curiel is that it undermines their argument that the next president should fill the vacant Supreme Court seat. By stalling the Supreme Court hearings, they are effectively saying that they want Trump to install the justice after he is elected. After all, he is now the presumptive candidate. But now Trump says Curiel, due to his hispanic heritage, can't be an impartial adjudicator. Trump's rhetoric (and repeated, after rebuke from leaders of his own party) reveals a desire to use his power to replace an independent judiciary with a judiciary that is subservient to his ends, to benefit his business interests, for example.
There was someone else, a few decades back, who destroyed a nation's independent judiciary to serve his personal agenda: His name was Adolf Hitler.