Washington (CNN) Republican Sen. Jeff Flake of Arizona condemned President Donald Trump's attacks against former White House aide Omarosa Manigault Newman as "unbecoming," just hours after Trump called the only African-American to have served in a senior role in the White House a "dog."

"This kind of language is unbecoming of a President of the United States," Flake, an Arizona Republican, wrote on Twitter Tuesday. "There is no excuse for it, and Republicans should not be okay with it."

Trump's reference on Tuesday morning to Manigault Newman as a "crazed, crying lowlife" and a "dog" is at best a sharp departure from the language typically employed by Presidents and at worst a reference that traffics in sexual and racial imagery. Trump has long denied being racist and has dismissed a claim made by Manigault Newman that he used a racial slur on the set of "The Apprentice." He's also invoked "dog" to insult non-African-Americans -- including Mitt Romney and former White House chief strategist Steve Bannon.

This kind of language is unbecoming of a President of the United States. There is no excuse for it, and Republicans should not be okay with it. https://t.co/0TLmRT7y22 — Jeff Flake (@JeffFlake) August 14, 2018

Trump's latest attack against Manigault Newman comes amid the release of her tell-all book "Unhinged: An Insider Account of the Trump White House," which contains several unflattering claims against the President and his staff. The White House has denounced Manigault Newman's book as "riddled with lies and false accusations."

Flake, who is not seeking re-election in 2018, has been a frequent critic of the President. Trump has responded in saying that Flake decided not to run again because his career is "toast" and had " zero chance of being elected."

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