After taking the floor, McConnell explained that Trump would sign the bipartisan bill averting a shutdown but also said that he would support Trump on a declaration of emergency in order for additional border security to be secured.
The Kentucky Republican said he'd spoken personally to the president to get the assurance about signing the bill.
The bill provides a little more than $1.3 billion for border fencing along 55 miles of the southern border, which is a mere fraction of what the president had requested.
Before the last shutdown, which started December 22, Trump had forecast he would sign a short-term funding measure to keep the government open while border security negotiations continued, but he had changed his mind.
Trump began weighing whether to declare a national emergency after Democrats refused to cave to his demand for $5.7bn in funding for the wall, prompting a 35-day government shutdown - the longest in USA history.
Now that it's looking likely that Trump will exercise that power to build his dream wall along the US border with Mexico, you may be wondering what, if anything, Congress can do to stop him.
That prospect seems unlikely, however, now that Senate Majority Leader Mitch McConnell has said he will support Trump's emergency declaration. "He will also be issuing a national emergency declaration at the same time", Mr McConnell said.
After the Senate votes on the bill Thursday, the legislation will be taken up by the House later Thursday afternoon. His fulminations about a border wall having failed to convince the legislative branch, which forged a deal that yielded less than a quarter of the funds the president demanded, he has decided by his planned emergency declaration simply to render the legislative branch irrelevant.
A congressional aide says the president could dip into $21 billion in unused military construction funds for building a wall.
But, if you can't do it, if the Congress rejects that, the president can't suddenly say, well, I didn't get my way, so I'm now going to go around the Constitution to find ways to fund it and get around the power of the purse.
It's still unclear exactly what powers the president will cite to declare some form of national emergency.
Look, our system of government was built on the president, if he wants appropriated funds, you go to the Congress, and you ask for those appropriated funds. No "emergency" or presidential powers will allow him to build the wall, ever, after he signs this bill.
Rep. Tulsi Gabbard (D-HI) said Trump's declaration would set a "very unsafe precedent" that would bring the country closer to "a dictatorship".
"I think it's ridiculous", Sen.
"Whether the president has the authority or not, it sets a risky precedent and places America on a path that we will regret", he said. Democrats also see victories in increased funding for humanitarian aid at the border, alternatives to detention and aid to Central America. The measure would provide more than 300 billion US dollars to fund the Department of Homeland Security and a range of other agencies through September 30, the end of the current fiscal year.
He would be able to divert money from existing military or disaster relief budgets to pay for the wall.