GOP senators see the former House lawmaker as an obstacle to striking deals on spending, including a stalled disaster relief package. The intraparty battle could spill over into high-profile debates on fiscal matters, such as raising the debt ceiling and avoiding another government shutdown.

Before joining the administration, Mulvaney was a founding member of the conservative House Freedom Caucus, which rose to prominence after the Tea Party wave of 2010 by opposing spending increases and the implementation of ObamaCare. Some GOP lawmakers worry that Mulvaney has ingrained the Freedom Caucus’s staunch conservative worldview to the White House, making it tougher to cut deals with Democrats.

“There is a feeling that the Freedom Caucus may be on the wane in the House, but it’s on the ascendency in the West Wing,” said one Republican senator, who requested anonymity to discuss colleagues’ frustration with Mulvaney.