Washington (CNN) The Justice Department issued a defense of President Donald Trump's controversial appointment of Matt Whitaker as acting attorney general Wednesday, offering several reasons why the appointment is consistent with the Constitution, federal statutes and past precedent.

The memo provides a robust legal justification for Whitaker's right to lead the Justice Department, which includes ultimate oversight authority over special counsel Robert Mueller.

In a 20-page memo addressed to the White House from DOJ's Office of Legal Counsel -- the chief legal interpreter of federal law for the administration -- the department concludes that:

* Whitaker's appointment is consistent with the plain terms of the Vacancies Reform Act, "because he had been serving in the Department of Justice at a sufficiently senior pay level for over a year" as the statute requires.

* Whitaker is only serving on a temporary basis, so he wasn't required to be confirmed by the Senate ahead of his selection last week, though the Justice Department acknowledged that situation had not come up in over a century. DOJ found that in 1866, a non-Senate confirmed assistant attorney general served as acting attorney general, before the department was founded.

Read More