At least four Republicans say they will defy president’s move on the Mexican border designed to fund his wall

Donald Trump’s declaration of an emergency on the Mexican border will be rejected by the Senate, the most senior Republican in the US upper chamber has admitted.

The Democratic-controlled House has already voted to reject the national emergency declaration. A rejection in the Republican-controlled Senate would send a powerful signal that Trump’s control of his own party may be slipping.

If his national emergency declaration is rejected, Trump could veto that rejection. A presidential veto can be overruled by both houses of Congress, but only with two-thirds majorities, which is unlikely, meaning the declaration is likely to stand in the end.

“I think 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 [of Representatives],” Mitch McConnell, the Senate majority leader, said.

The remarks from McConnell – who fell in line behind Trump on the issue despite his misgivings – came after Rand Paul became the latest Republican senator to say he would reject the declaration, which the president made to circumvent Congress and funnel billions of extra dollars towards erecting his proposed border wall.

Susan Collins of Maine, Lisa Murkowski of Alaska and Thom Tillis of North Carolina have also announced they will defy Trump. Assuming all 47 Democratic senators and their independent allies also vote against Trump, that will give opponents of the emergency declaration 51 votes and the majority they need.

The Senate vote is expected next week.

On Monday, Paul said he thought “at least 10” Republican senators were prepared to vote against Trump.

McConnell, who has worked closely with Trump on the tax system overhaul, the selection of conservative judges and other issues, acknowledged that he had advised the president against making the declaration.

The Senate leader said he was worried the move would set a precedent and allow future Democratic presidents to make similar moves for their own purposes.

“That’s one reason I argued, obviously without success, to the president, that he not take this route,” McConnell said.

Some lawmakers opposed to the emergency declaration say it tramples on Congress’s constitutional power to control spending, or are concerned Trump would siphon money from projects in their home states for barrier construction.

At a Republican dinner last weekend in Kentucky, Paul said: “I can’t vote to give the president the power to spend money that hasn’t been appropriated by Congress.

“We may want more money for border security, but Congress didn’t authorise it. If we take away those checks and balances, it’s a dangerous thing,” Paul added, according to the Bowling Green Daily News.

Under the declaration, Trump would divert $3.6bn (£2.7bn) from military construction to erect more border barriers, and is invoking other powers to transfer an additional $3.1bn to fund construction.

Lawsuits have been filed to try to derail the declaration, which could at least prevent Trump from getting the extra money for months or longer.

Paul said he spoke to Trump on Sunday night and detected no signs of backing down. “I think he’s made his decision,” he told reporters.

The Associated Press contributed to this report