If Republicans who favor more Pentagon spending help confirm President Donald Trump’s nominee for budget director, they will be holding their noses when they do it.

Among GOP lawmakers who want to increase funding for defense, reservations run deep about Rep. Mick Mulvaney to be director of the Office of Management and Budget.

Those concerns exploded into the open Tuesday at one of Mulvaney’s two Senate confirmation hearings.

“All I can say to you sir is, I’m deeply concerned about your lack of support for the military,” said John McCain, R-Ariz., the Armed Services Committee chairman.

When Mulvaney could not recall some of his past votes, which have included proposals to reduce defense spending and bring U.S. troops home from Afghanistan and Europe, McCain shot back: “I tell you, I would remember if I voted to cut our defense the way you did. Maybe you don’t take it with the seriousness that it deserves.”

The exchange dramatized a larger conflict on defense spending, one that is likely to be a source of great political tension going forward.

Mulvaney, a South Carolina Republican, has served in the House since 2011. He has shown fealty to two views that pose problems for advocates of boosting the military’s accounts. He has opposed increases in defense spending that aren’t accompanied by cuts in non-defense spending, and he has opposed using the war budget for projects unrelated to war.

If Mulvaney insists that defense budget requests move core, ongoing military programs out of the war budget and back to the so-called base budget, then the statutory budget caps on the base budget will have to be raised by about $30 billion a year, on top of whatever increase the hawks want.

Raising the caps that much will require a heavy political lift.

The question among Republicans is not whether to increase Pentagon spending so much as how to pay for it. Defense hawks like Rep. Mac Thornberry, R-Texas, and McCain want to increase defense spending and—like the deficit hawks—they would prefer to also reduce domestic spending and keep the war budget only for war programs.

However, the difference is that the defense boosters, unlike Mulvaney, do not insist that every single dollar of increase for the defense base budget must be offset by a dollar cut elsewhere. Nor do they insist, as Mulvaney has up to now, that not one penny of the war account be spent on base-budget programs.

Democrats, meanwhile, are generally either opposed to increasing the defense budget, unless paired with corresponding domestic spending boosts, or don’t want increases paid for via cuts to non-defense programs.

In the new Congress, Democrats may no longer be able to ensure, as they generally have to this point, that any dollar increase in the defense budget cap is matched by a dollar increase in the non-defense cap. Nonetheless, Democrats will still oppose any Trump/Mulvaney plan to pay for Pentagon spending hikes via cuts to domestic programs. Some Republicans will agree, if only quietly.

Because of the Senate’s rules, Democrats still retain clout. Republicans in that chamber have 52 seats. So if they stick together, Republicans will need eight Democrats to support potentially deep cuts to domestic programs—a very tough sell.

The idea of moving scores of billions from butter to guns is “complete fantasy and dead on arrival,” said Mackenzie Eaglen, a former GOP aide in the Senate who is now a defense budget expert with the conservative American Enterprise Institute. Eaglen made her remarks at the Center for Strategic and International Studies on Jan. 23.

Plus, to the degree Trump recommends cutting taxes and spending more on infrastructure, it will devour still more resources that defense hawks believe the Pentagon could use and may further increase the deficit, strengthening the hand of budget hawks.

When all is said and done, defense spending will probably continue to go up anyway—and in the same fashion it has in the last several years: through a combination of slightly raising the defense caps (and maybe the non-defense ones) and continuing to use the war account to some degree, experts say.

But it will be extremely difficult to boost Pentagon spending by the tens of billions of dollars a year that defense hawks would like. And Mulvaney’s deeply held policy views could make doing so even harder.

— Ryan McCrimmon contributed to this story.