"Trump will luxuriate in his victory. But the personal victory for him is a legal assault on the Constitution," Napolitano wrote in the op-ed on FoxNews.com. "The president has taken an oath to preserve, protect and defend the Constitution. Instead, he has trashed it."

"How? By manipulating Senate Republicans to bar firsthand evidence and keep it from senatorial and public scrutiny, Trump and his Senate collaborators have insulated him and future presidents from the moral and constitutional truism that no president is above the law," he continued. "Somewhere, Richard Nixon is smiling.”

Napolitano said on Fox News in December that he would have voted to impeach Trump if he were a member of the House of Representatives.

Senate Republicans voted to acquit Trump of the abuse of power charge in a 52-48 vote, with Romney joining all Democrats in voting to convict Trump. The second article, accusing Trump of obstruction of Congress, failed in a party-line 53-47 vote.

During appearances Thursday at the National Prayer Breakfast and in extended remarks delivered at the White House, Trump held up several newspapers with headlines of his acquittal.