Washington (CNN) A Democratic senator said Thursday that White House chief of staff John Kelly should be held accountable if he knew about domestic abuse allegations against a White House staffer.

Senior aides to President Donald Trump knew for months about allegations of domestic abuse levied against top White House staffer Rob Porter by his ex-wives, even as Porter's stock in the West Wing continued to rise, multiple sources told CNN on Wednesday.

Porter denied the allegations but resigned on Wednesday. The White House declined to comment on Wednesday when asked about Kelly's knowledge of the allegations against Porter.

"If John Kelly is covering this up, he needs to be held accountable," Sen. Jon Tester, D-Montana, told CNN's "New Day," adding, "He better have a really good reason. Otherwise, he's gone, too." The chief of staff position is appointed, not confirmed by the Senate, so removing him would be up to Trump.

By early fall, it was widely known among Trump's top aides -- including Kelly -- both that Porter was facing troubles in obtaining a security clearance and that his ex-wives claimed he had abused them. No action was taken to remove him from the staff.

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