The court's actions, the attorney general said in a speech to the conservative Heritage Foundation, represent an improper attempt "to hold a trial over the inner-workings of a Cabinet secretary's mind."
With his remarks, Sessions waded into a simmering legal dispute that may ultimately be resolved by the Supreme Court, which solidified its conservative majority with the recent addition of Justice Brett Kavanaugh.
The conflict centers on a judge's order that Ross may be deposed by lawyers challenging whether a question on citizenship legally can be included on the census. Plaintiffs in two lawsuits, including more than a dozen states and big cities, have sued, saying the question will discourage immigrants from participating in the census.
The judge, Jesse M. Furman, has said Ross can be questioned about how the citizenship inquiry was added to the census because he was "personally and directly involved in the decision, and the unusual process leading to it, to an unusual degree." A New York-based federal appeals court backed Furman's ruling last week, but Supreme Court Justice Ruth Bader Ginsburg issued a temporary stay.
In prepared remarks, Sessions decried the court order as but one example of judges improperly scrutinizing "internal deliberations" of an executive branch decision. He said judges too often are behaving like "roving inspectors general for the entire executive branch."
"The words on the page don't have a motive; they are either permitted or they are not," Sessions said. "But the judge has decided to hold a trial over the inner-workings of a Cabinet Secretary's mind."
Sessions said the Trump administration plans to continue challenging efforts by courts to expose private deliberations of government agencies or to force the questioning of officials under oath.
"Subjecting the executive branch to this kind of discovery is not acceptable, it's just not," he said. "And we intend to fight it, and we intend to win."
"That's why we're taking these cases to the appellate courts and to the Supreme Court," the attorney general added. "These actions are disrupting orderly governmental functions and placing great cost on the American people."
The attorney general's remarks came just days after Ross, in a Justice Department filing, said he now remembers speaking with former senior White House adviser Steve Bannon last year about adding a citizenship question to the census. He also recalled discussing the matter with Sessions in the spring of 2017 and at other times, Justice Department lawyers wrote in the filing.
Ross had previously testified under oath that he was unaware of discussions between himself and anyone in the White House on the subject.
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