The post has been updated.

Either the White House has a highly secret plan to revive the line-item veto, or President Trump has again forced those around him to try to explain one of his fanciful ideas.

Treasury Secretary Steven Mnuchin appeared on “Fox News Sunday” this weekend in part to make the case for Trump getting a line-item veto (i.e., the ability to eliminate individual provisions in bills that are passed by Congress). Trump floated the idea Friday while grudgingly signing Congress's 2,200-page omnibus spending bill.

But host Chris Wallace noted that there's a big problem: the Constitution. He pointed out that the line-item veto was struck down by the Supreme Court in the 1990s.

Mnuchin's response? To suggest there was a workaround to installing a new line-item veto. Here's the exchange:

MNUCHIN: As you heard him say, he's not planning on [signing an omnibus] again. I think they should give the president a line-item veto. These things should be looked at —

WALLACE: But that's been ruled unconstitutional by the Supreme Court, sir.

MNUCHIN: Well, again, Congress could pass a rule, okay, that allows them to do it. But —

WALLACE: No, no, sir, it would be a constitutional amendment.

MNUCHIN: Chris, we don't — we don't need to get into a debate in terms of — there's different ways of doing this. My comment is, it's clear what happened.

The White House doubled down on Mnuchin's comments Monday, with deputy press secretary Raj Shah saying, “There are certain things being discussed with respect to House and Senate rules.”

Experts are dumbfounded as to what that could be.

“Mnuchin probably doesn’t know that there was a real-life experiment in the 1990s that answered this question,” said Robert Spitzer, an expert on presidential vetoes who has written about President Bill Clinton's failed effort to use a line-term veto. Spitzer said Clinton used the line-item veto on 10 bills and about 80 different provisions in 1997 before the Supreme Court ruled it was unconstitutional in the 1998 case Clinton v. City of New York. Overturning that ruling would require two-thirds majorities of both chambers and ratification by 38 out of 50 states. In other words, Congress couldn't do that on its own.

But is there any way Trump could get line-item veto authority without an amendment? Spitzer and others strained to imagine one.

“It is theoretically possible that an imaginative lawyer might invent a mechanism that would pass constitutional muster, but I know of no such invention to date,” Spitzer said. “The other thing is that the court was pretty emphatic in its decision that any kind of item veto would have to be granted through constitutional amendment, so that by itself pretty much slams the door on Mnuchin’s speculation.”

Added Roger Hartley, a constitutional amendment expert at Catholic University: “I could not imagine what ‘different ways’ he was thinking that are available to the Congress to enact constitutional line-item veto legislation absent a constitutional amendment.”

Another amendment expert, John Vile at Middle Tennessee State University, said the only other way would be “for the Supreme Court to reverse the decision when confronted with a similar bill.” But the court's decision was 6-3, he added, and it has signaled no desire to revisit the issue. (Plus, only one of the three dissenters, Stephen G. Breyer, remains on the court.)

Some are floating the idea that legislation like the omnibus could be broken down into many thousands of smaller bills and passed at the same time — Congress has extensive authority over its own rules — allowing Trump to veto the ones he doesn't like. This would not be a line-item veto, strictly speaking, and it would have to pass legal muster and be approved by Congress. But it would certainly be a novel approach.

It's possible the White House has devised a secret plan to craft a de facto line-item veto through congressional rules, but it also seems possible Trump was floating something unreasonable for which they now have to answer.

Trump has shown repeatedly that he isn't exactly tuned in to the legislative process or the limits of his authority. After failing to pass an Affordable Care Act replacement, for example, Trump began pushing for the Senate to get rid of the 60-vote threshold to end filibusters — despite that legislation not being subject to the filibuster. According to Rep. Mark Sanford (R-S.C.), Trump told lawmakers a few months ago that he wanted to protect “Article XII” of the Constitution, which doesn't exist. Trump at one point expressed surprise that Abraham Lincoln was a Republican. And former Trump campaign aide Sam Nunberg recalled in Michael Wolff's book that he tried to teach Trump about the Constitution, and “I got as far as the Fourth Amendment, before his finger is pulling down on his lip and his eyes are rolling back in his head.”

We'll see how thoroughly thought through this all has been.