This article has been updated.

President Trump crossed the Mississippi on Wednesday to launch a new (though somewhat vague) push for a reform of the tax code. He was in Missouri, a state that (as he pointed out on Twitter in announcing the visit) he won by a lot last year. While talking about this new legislative push, he couldn’t help but predict that the state’s Democratic senior senator would not be on board.

“We must lower our taxes. And your senator, Claire McCaskill, she must do this for you,” Trump said. “And if she doesn’t do it for you, you have to vote. Her. Out. Of. Office.”

The crowd cheered. But former Barack Obama adviser Dan Pfeiffer noted on Twitter that this was not just a normal comment for a president to make at an official presidential event.

Turns out, Pfeiffer’s right.

“That is a real problem,” said Larry Noble, senior director of the Campaign Legal Center.

Trump’s “not supposed to do any electioneering when he’s on government business,” Noble explained. “Calling for the defeat of somebody is electioneering and is a campaign statement, and therefore should only take place at a campaign event.” As a result, Pfeiffer’s right: The Republican National Committee or the Trump 2020 campaign — which has up and running since January — should cover part of the cost of Wednesday’s event.

This rule, dictated by the Federal Election Commission, usually comes up when an elected official is at an event as part of their official duties and is asked by a member of the media about an upcoming election. Then, too, the official is expected to demur from offering a response.

Noble pointed out that the response from Trump’s team may be that the president was saying if-this-then-that — that it was not an explicit call for McCaskill’s defeat, but a conditional one. He didn’t think this was a winning argument. It also seems unlikely that this was a narrow path that Trump decided on the spur of the moment to walk.

The resolution to this is as simple as the RNC or Trump’s campaign cutting a check. But to the point of the first part of Pfeiffer’s tweet, this is also an unusual line for a president to cross.

It’s worth remembering, though, that Trump’s spent an awful lot more time behind microphones as a candidate than as a president. And old habits die hard.

Particularly for Donald Trump, it seems.

Update: Press secretary Sarah Huckabee Sanders addressed this issue briefly during a brief discussion on Air Force One.

“It’s not a political threat when you ask a member of Congress to do their job,” Sanders said.