Washington (CNN) Donald Trump is claiming that a judge with Mexican heritage who is overseeing a lawsuit against Trump University has an "inherent conflict of interest" given the presumptive GOP nominee's rhetoric on immigration.

Trump's comments to The Wall Street Journal on Thursday escalate a line of attack against U.S. District Court Judge Gonzalo Curiel, who last week ordered parts of internal documents, including "playbooks" regarding running the enterprise, to be released as part of a lawsuit against Trump University.

The documents were released in response to a request by The Washington Post.

"I'm building a wall. It's an inherent conflict of interest," Trump told the Journal.

Speaking to CNN's Anderson Cooper on Thursday night, CNN senior legal analyst Jeffrey Toobin said Curiel's background was not a reason for him to step down from the case.

"The ethnicity of the judge is not in it of itself a grounds for recusal, ever," Toobin said. "And that is a claim that Trump -- under the First Amendment -- is certainly allowed to make, but it is completely wrong under the law."

Recently, Trump and his surrogates have attacked Curiel, a federal district judge in the Southern District of California who was born in Indiana

"I have a judge who is a hater of Donald Trump, a hater. He's a hater," Trump said on Friday in San Diego, saying Curiel is, "We believe, Mexican, which is great."

Curiel has allowed for the "playbooks" about Trump's embattled namesake school to be opened. He said there is now public interest in them since Trump "became the front-runner in the Republican nomination in the 2016 presidential race, and has placed the integrity of these court proceedings at issue."

Trump's campaign aides have also criticized Curiel as a member of the former La Raza Lawyer's Association, which is a leading Latino lawyers group in California.