Donald Trump on Thursday escalated his attacks on the federal judge presiding over civil fraud lawsuits against Trump University, amid criticism from legal observers who say the presumptive GOP presidential nominee’s comments are an unusual affront on an independent judiciary.

In an interview, Mr. Trump said U.S. District Judge Gonzalo Curiel had “an absolute conflict” in presiding over the litigation given that he was “of Mexican heritage” and a member of a Latino lawyers’ association. Mr. Trump said the background of the...