Senator Susan Collins of Maine said such a declaration was “of dubious constitutionality,” while Senator Marco Rubio of Florida called it “a bad idea.” Senator Rand Paul of Kentucky said, “We have a government that has a Constitution that has a division of power, and revenue raising and spending power was given to Congress.”
Still, other Republicans backed Mr. Trump, including an ally, Senator Lindsey Graham of South Carolina, as well as Senator Shelley Moore Capito of West Virginia, who faces re-election next year. “He feels that declaring a national emergency gives him that flexibility that he wants and needs, so I’m supportive of that,” Ms. Capito said of the president.
“For goodness’ sake, we tried for 35 days in December and January, right, to get the Democrats to do what everyone knows needs to happen except Democrats,” said Representative Jim Jordan, Republican of Ohio. “I support the national emergency declaration 100 percent.”
Among the Republicans who privately warned Mr. Trump against an emergency declaration was Mr. McConnell, who expects that House Democrats will pass a nonbinding resolution disapproving it in a form that the Republican leader cannot block from a floor vote. At least five or six Republican senators are likely to vote against the president, making a majority along with the Democrats.
A person familiar with the discussions said that Mr. McConnell said that he has warned Mr. Trump that he has less than two weeks to try to persuade wavering Republicans to support his national emergency effort, otherwise he will face the prospect of a bipartisan rebuke by Congress.
The spending legislation headed to passage on Thursday includes the seven remaining bills to keep the remainder of the government open through the rest of the fiscal year at the end of September. House and Senate negotiators unveiled the 1,159-page bill on Wednesday just before midnight, leaving little time for lawmakers to actually digest its contents.
The border security compromise tucked into the bill is perhaps the most stinging legislative defeat of Mr. Trump’s presidency. It provides $1.375 billion for 55 miles of steel-post fencing, essentially the same deal that Mr. Trump rejected in December, triggering the shutdown, and far from the $5.7 billion he demanded for more than 200 miles of steel or concrete wall.