WASHINGTON (AP) — Conservative leaders are rallying around White House acting chief of staff Mick Mulvaney, who is facing increasing scrutiny for his response to Democrats' impeachment probe into President Donald Trump.

In an open letter set for release Thursday afternoon, conservatives hail Mulvaney as "the most successful chief of staff in this administration" and advocates for Trump giving him the role — which he has occupied since January — on a permanent basis.

Mulvaney came under fire from Trump allies after he appeared to acknowledge that Trump held up military aid to Ukraine partly for political reasons. He then later denied that he had said it.

The signatories of the letter include the leaders of conservative groups like the Club for Growth, the Family Research Council, and Heritage Action.