President Trump is mostly getting his way with cabinet choices, but may be facing a major battle over his nominee to run the Office of Management and Budget (OMB), Rep. Mick Mulvaney (R – SC), whose fiscal conservatism doesn’t at all sit well with hawks looking for runaway military spending.

Part of the Tea Party wave of elections in 2010, Mulvaney has repeatedly been among a handful of Republican Congressmen who has resisted efforts to expand military spending, trying to freeze spending levels on the grounds that the US just can’t afford to continue its spending surge since 9/11.

That appears to put him at odds with President Trump, who has been hyping the idea of massive increases in military spending. The Trump transition team argues that Mulvaney doesn’t oppose higher military spending, so long as it is offset by cuts elsewhere and done in a fiscally responsible way.

US military spending and “fiscally responsible” rarely go hand in hand, however, and the fact that Mulvaney ever voted for anything short of the biggest spending options on the Pentagon has made him a lot of enemies on the Hill, many of whom are likely to oppose having him anywhere near the budget.

Sen. Rand Paul (R – KY) praised the selection of Mulvaney, calling him a “friend and great conservative leader,” while former OMB Director David Stockman said Mulvaney was a “great choice” who hadn’t supported former House Speaker John Boehner’s “budget betrayals.”

The OMB job under President Trump is going to be a difficult one, trying to help deliver on Trump’s promises of major savings on programs like the F-35, while trying to find room in the budget to fit all the substantial spending pledges Trump made during the campaign.

Mulvaney’s fiscal conservatism gives him credibility, and reason to believe he’d be trying to deliver a budget in good faith that doesn’t explode the debt. That is also reason for hawks to fear him, and will doubtless lead to some tension around his confirmation.