Donald Trump at a rally with James Mattis, his pick for defense secretary. AP Defense Secretary Jim Mattis is "not happy" the White House is arguing with him over who should take the top job in the Pentagon's policy shop, Foreign Policy reports.

According to FP, Mattis wants Mary Beth Long, a George W. Bush-era Pentagon official, to come aboard as undersecretary of defense for policy, but the White House is insisting he go with Mira Ricardel, a Trump transition team member, who also served in the Bush administration.

This doesn't seem to be a fight over qualifications, however.

FP reports that one of the issues the Trump administration has is that Long was among more than 100 Republican national-security leaders who signed an open letter in March saying then-candidate Donald Trump was "fundamentally dishonest" and unfit for office, as well as listing other complaints.

Long later walked back the criticism and dropped her "Never Trump" position, telling NPR in November that it was "a moral and civic duty to get behind this president."

Still, the report from FP suggests an apparent blacklist for Republicans who openly opposed Trump before the election.

Some other "Never Trump" Republicans told The Washington Post they were not being called for national-security positions likely because of their opposition. Former State Department official Eliot Cohen said White House jobs were being viewed as "lollipops, things you give out to good boys and girls."

Theresa Whelan is the acting undersecretary.

The undersecretary of defense for policy isn't the only unfilled job at the Pentagon's policy office. The No. 2 spot, principal deputy undersecretary of defense for policy, is vacant, as well as supporting roles covering cyber, space, Afghanistan-Pakistan, Russia-Ukraine, and nuclear and missile defense policy.