The resolution passed the House last week 245-182 with the help of 13 Republicans. To pass the Senate, it needs only four Republicans to vote for it, plus all the Democrats.
"What is clear in the Senate is there will be enough votes to pass the resolution of disapproval, which will then be vetoed by the president and then in all likelihood the veto will be upheld in the House," he said at a Monday press conference in the Capitol.
Trump declared a national emergency over security at the border to access funding to build a wall.
"I would literally lose my political soul if I decided to treat President Trump different than President [Barack] Obama," Paul wrote in a Fox News editorial. "I support President Trump. I supported his fight to get funding for the wall from Republicans and Democrats alike, and I share his view that we need more and better border security. However, I cannot support the use of emergency powers to get more funding, so I will be voting to disapprove of his declaration when it comes before the Senate."
If the Senate supports the block, it would be vetoed by Trump and return to Capitol Hill where Democratic lawmakers would attempt to override the veto, which would be the first of Trump's presidency. It would require a two-thirds majority vote of both houses of Congress to overturn the veto. It's believed Democrats don't have the votes in the House or the Senate to override, meaning the issue would then be settled in federal courts.
Trump mentioned the Senate vote during his two-hour speech at the Conservative Political Action Conference last weekend, praising House Republicans for taking a stand.
"And a lot of people talk about precedent. Precedent. That if we do this, the Democrats will use national emergency powers for something that we don't want," Trump said. "They're going to do that anyway, folks. The best way to stop that is to make sure that I win the election. That's the best way to stop that."
Sen. John Kennedy, R-La., said, the emergency declaration is within Trump's rights.
"The president's not exercising any power that Congress didn't give him," Kennedy said. "Had Congress done its job instead of playing politics, he wouldn't have to do it."
Democrats say Trump is trying to do an "end run around the Constitution."
"This is a president who is grasping for power, and he has to be reined in," said Sen. Tom Udall, D-N.M.
The Senate is required to vote on the resolution before March 15.