CNN  — 

President Donald Trump will sign a bill to avert another government shutdown, but also issue a national emergency declaration to build a wall on the US southern border, according to Senate Majority Leader Mitch McConnell, replacing lawmakers’ most pressing problem with one that strikes at their power to spend money.

For his first two years in office, Trump enjoyed Republican control of Congress and was still unable to get what he wanted on border security, his signature campaign issue. The likelihood of that dropped even further when Democrats won control of the House of Representatives in the November elections.

Trump’s first reaction to that result was to fight Congress, shutting down parts of the government from December 22 until January 25. Now, it appears he’ll go around it.

Republican and Democratic lawmakers reacted to McConnell’s announcement with alarm.

“I think it’s a mistake,” said Sen. Susan Collins, Republican of Maine. “The National Emergencies Act was contemplated to apply to natural disasters or catastrophic events such as the attacks on our country on 9/11. For the President to use it to re-purpose billions of dollars, that Congress has appropriated for other purposes that has previously signed into law, strikes me as undermining the appropriations process, the will of Congress and being of dubious constitutionality.”

Before Thursday, McConnell made clear that he was also concerned of the prospect that the President would declare a national emergency to build a border wall. On January 10, in the middle of what became the longest US government shutdown ever, McConnell told The New York Times magazine, “I hope he doesn’t go down that path.”

And on January 29, during yet another round of negotiations over border security, McConnell told reporters, “I am for whatever works – which means avoiding a shutdown, and avoiding the President feeling he should declare a national emergency.”

“Exactly how to do that, as you all know, has been quite challenging,” he added.

But on Thursday, McConnell said that Trump supported the bill and he had “indicated” that he would support the President’s national emergency declaration. That announcement paved the way for the Senate to overwhelmingly pass a bill that funds 25% of the federal government, including around $1.375 billion for barriers, which is much lower than Trump’s push for $5 billion to fund the border wall. The House is expected to pass it Thursday night, sending it to the President for his signature.

When asked by reporters about his latest call with Trump, McConnell said he “urged” the President to sign the bill. “That was my focus,” he said.

If the President proceeds with the declaration, it’ll likely be challenged in court and by Democrats in Congress, as critics have argued that Trump cannot use the national emergency authority to free up taxpayer funds and build the border wall he has long promised to his political supporters.

Even among members of his own party, support for the President was tepid at best. Veteran Republican senators raised various doubts about using his emergency powers to try to build the wall. Some said they wanted to wait and see exactly what Trump will declare and are skeptical he will do more than use existing executive authority.

“I do (support that decision) if that’s what it takes to do it,” said Sen. Jim Inhofe, the Republican chairman of the Armed Services committee. “I just don’t want it coming out of defense.”

“But what I want doesn’t seem to make that much difference,” he added.

Other rank-and-file Republicans, including Rep. Cathy McMorris Rodgers of Washington and Sen. Mike Rounds of South Dakota, said they were worried that it would set a new precedent for a Democratic president to set liberal policies without engaging Congress.

In an interview, Rounds asked, “What if somebody else thinks that climate change is a national emergency – then what will they do and how far will they go?”

Sen. John Cornyn of Texas said an emergency declaration is “not a very practical solution to the problem” since the funding would get “tied up” if there’s lawsuit.

According to federal law, Congress can rescind a presidential emergency declaration by passing a joint resolution. In the likely event that such a bill would be vetoed by the President, Congress could then override it with a two-thirds majority in the Senate and the House.

If the House passed it, then the Senate would be required by law to vote on the measure within 18 days, leaving McConnell without a way to stop a vote on the floor.

Democratic leaders said the President’s action would be a threat to the separation of powers. House Judiciary Committee Chairman Jerry Nadler said he would “fully support” a joint resolution to rescind the emergency declaration and intended to “pursue all other available legal options.”

House Speaker Nancy Pelosi and Senate Democratic Leader Chuck Schumer said “the President’s fearmongering” doesn’t make it an emergency.

“He couldn’t convince Mexico, the American people or their elected representatives to pay for his ineffective and expensive wall, so now he’s trying an end-run around Congress in a desperate attempt to put taxpayers on the hook for it,” they said in a statement. “The Congress will defend our constitutional authorities.”

CNN’s Michael Warren, Clare Foran, Priscilla Alvarez and Tammy Kupperman contributed to this report.