(CNN) A Clinton-era White House press secretary suggested Sarah Sanders ought to issue an ultimatum to President Donald Trump: Tell me the truth or I'll resign.

"I think at this point, I'd go into the Oval Office and tell the President that if he lies to me again, I have to quit," Joe Lockhart, who is a CNN political commentator, told anchor Ana Cabrera on "CNN Newsroom" Saturday.

"I think she runs the risk, it's at the stage where she's complicit in the lies, even though she may not have the best information" Lockhart continued, adding that Sanders "now knows for certain that the President is a liar."

"We now know, without any shadow of a doubt, that the President will say whatever is in his interest at that moment and has no regard for the truth. So she knows this," Lockhart added.

"Going forward, she has to make clear she's not going to carry his water anymore," he said.