Mr. Paul said as much in an opinion piece published late Sunday on Fox News’s website, and again on Monday.
“It’s not like I sat up one day and said, ‘Where do I want to separate myself from President Trump?’ There are political risks to separating yourself from your own party’s president,” he said, adding, “To me, it isn’t even about immigration, it isn’t about a Republican or Democratic president.”
“It’s about Congress versus the president and where the power should be distributed,” he concluded.
In his opinion piece outlining his vote, Mr. Paul went so far as to speculate that Justices Neil M. Gorsuch and Brett M. Kavanaugh — both conservative judges appointed by Mr. Trump to the Supreme Court — would join the majority in ruling against the administration and the national emergency declaration.
“Without question, the president’s order for more wall money contradicts the will of Congress and will, in all likelihood, be struck down by the Supreme Court,” Mr. Paul wrote. “In fact, I think the president’s own picks to the Supreme Court may rebuke him on this.”
Mr. Trump’s emergency declaration — the first time a president has invoked powers under the National Emergencies Act after Congress denied funds — is a particularly thorny issue for Mr. McConnell.
Like many of his conservative colleagues, he warned the president against setting a precedent that future Democratic presidents could seize as a means for carrying out stringent gun control policies or climate change controls. But it was Mr. McConnell, in a speech on the Senate floor, who announced Mr. Trump’s intent to declare a national emergency — and his support for the president’s decision.
On Monday morning, the White House dispatched Zach Parkinson, the deputy director of government communications, to meet with Republican communications aides, according to two people in the meeting, who asked for anonymity to discuss a private meeting. Mr. Parkinson asked the aides to “keep their powder dry” ahead of the vote, and urged them to have their senators reach out to the White House Office of Legislative Affairs with concerns.