Senate Republicans are warning President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE ahead of his State of the Union speech against using a national emergency declaration to build the U.S.-Mexico border wall.

GOP lawmakers warned that declaring a national emergency would be met with resistance on Capitol Hill, where Congress could try to block Trump by using a resolution of disapproval.

"There's a lot of reservations in the conference about it and I hope they don't go down that path," Thune told reporters.

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Congress has until Feb. 15 to get a deal on funding for the border wall and to prevent a second shutdown, which would impact roughly a quarter of the government. The president is demanding $5.7 billion for the wall; Democrats have rejected money for a concrete wall but signaled some openness to fencing or other barriers.

"I wouldn't prefer one. I believe we should continue to work toward the legislative solution," he said. "I do believe that the president's probably got the power under the Constitution and maybe under the statue to do that, but I would rather us do it."

Trump has refused to rule out declaring a national emergency if Congress isn't able to reach a deal on wall funding, despite public pushback from Republicans including McConnell.

He hinted to reporters on Friday that they should "listen closely" to his State of the Union speech.

McConnell reportedly warned Trump that a resolution blocking his emergency declaration would be able to get enough GOP votes to pass, forcing the president to issue his first veto.

"I don't think the intent was for it to be used in this kind of situation. And as a member of the Senate I'm very concerned if the president believes that he can reallocate or repurpose appropriations for which we have designated very specific purposes," she said.

"It also is in my judgement of dubious constitutionality," Collins added. "So I don't really think it gets him to the goal."

Republicans have tried to stick closely to Trump during the months-long fight over the border wall, which remains a potent issue among the party's base. But there have been growing fractures about shutdown strategy after GOP senators became increasingly anxious during the 35-day partial funding lapse.

Trump would likely have some GOP support if he declared a national emergency.

John Kennedy John Neely KennedyMORE Sen.(R-La.) has repeatedly told reporters that he doesn't think it would "be the end of Western civilization," while also saying last week that he didn't know how he would vote on an effort to block the declaration.

"It seems to me that he's gonna have to go it alone, but there could be a war within the Republican party over the wall," Graham said.

He added that "any Republican who denies the president the ability to act as commander in chief, you're going to create a real problem within the party."