President Trump broke the law when he appointed Matthew Whitaker to replace Attorney General Jeff Sessions on Wednesday, lawyers Neal Katyal and George Conway argue in a New York Times op-ed Thursday.

The problem, they say, is that Whitaker was not confirmed by the Senate. They argue that that violates the Constitution’s Appointments Clause, which they say requires all “principal officers” of the government to be Senate-confirmed.

Indeed, it was very strange that Trump bypassed the next in line for the post, Deputy Attorney General Rod Rosenstein, and slotted in Whitaker instead. Rosenstein had been confirmed by the Senate for the deputy job.

But Whitaker was serving as Sessions’s chief of staff, which is not a Senate-confirmed post. Before that, he’d been out of government for eight years. That makes his appointment as the acting head of a Cabinet agency highly unusual, at the very least — and unconstitutional according to Katyal and Conway.

“It’s illegal. And it means that anything Mr. Whitaker does, or tries to do, in that position is invalid,” they write.

The argument may seem unsurprising coming from Katyal, who’s a Democrat and was President Obama’s solicitor general. But the fact that George Conway helped write the piece will certainly raise eyebrows, since he is of course the husband of top White House staffer Kellyanne Conway.

George Conway’s frequent public criticisms of Trump and his administration have been much noted, but here he is outright calling a major Trump appointment illegal, on an issue for which the stakes for the president and his family are enormously high.

Whitaker’s appointment spurred fears that Trump was installing a loyalist to take control of and perhaps crush special counsel Robert Mueller’s Russia investigation. And this new argument could be the opening volley in some eventual court battle over Whitaker’s authority.

Separately, Georgetown law professor Marty Lederman wrote a rundown of legal issues surrounding Whitaker’s appointment, and he categorized this issue less definitively as a “possible” constitutional problem. There’s little precedent for a situation like this, Lederman wrote, so “it’s technically an open question.”

You can read Katyal and Conway’s full op-ed at this link.