Sen. Pat Toomey voted last week against President Trump’s emergency border declaration, but he suggested Sunday that doesn’t necessarily mean Mr. Trump’s attempt to shift money is illegal.

He said his vote was meant to be a defense of Congress’s power to say where taxpayers’ money should be spent. But he said the courts may well side with Mr. Trump.

“I’m not sure it is straight up an illegal act,” Mr. Toomey told NBC’s “Meet the Press” on Sunday. “There is a plausible argument for the constitutionality.”

Mr. Toomey was one of 12 GOP senators who broke with Mr. Trump and joined Democrats in a 59-41 vote to overturn the February declaration of a border emergency, which Mr. Trump issued in order to shift money from Pentagon construction accounts toward building his border wall.

The president on Friday vetoed that measure.

Several states and activist groups have taken to the courts, suing to stop the declaration. Among their arguments are claims that Mr. Trump has broken the Constitution by rearranging the money, and that there is no border emergency so his declaration is illegal anyway.

When asked if the Supreme Court should take up the issue, Mr. Toomey demurred.

“The courts may very well end up siding with him,” he said.

Acting White House Chief of Staff Mick Mulvaney said Sunday he wanted those 12 GOP senators who voted against the president to identify what they’re doing to solve the border crisis.

“It’s up to them to go back and tell their voters why it was more important to overturn the president than it was to secure the southern border,” Mr. Mulvaney said during an appearance on “Fox News Sunday.”

“This was a border security issue,” he added.