The president regularly warns of an “invasion” of immigrants entering the U.S. from its southern border, despite studies showing illegal border crossings have significantly dropped over the last decade.

“He’s never vetoed appropriations legislation and now he wants to declare an emergency to do something Congress has already debated and discussed,” Amash said.

He added that Trump could have vetoed past spending bills if he truly felt there was a border security crisis that lawmakers needed to address.

“We have a system of separation of powers under our Constitution,” Amash, a member of the House oversight committee, said on CNN’s “State Of The Union.”

Rep. Justin Amash (R-Mich.) on Sunday bashed President Donald Trump ’s decision to unilaterally declare a national emergency to secure funding for his long-promised wall along the U.S.-Mexico border.

Rep. @JustinAmash , one of 13 Republicans to oppose Trump’s emergency declaration: “The President doesn’t get to decide that he can override Congress simply because Congress doesn’t do what he wants … If there were an emergency...there would be a lot more consensus” #CNNSOTU pic.twitter.com/X1Vb3702OQ

Trump allowed a partial government shutdown to start in late December after Democrats refused to allocate $5 billion toward his border wall in a federal spending bill. The budget impasse continued for 35 days ― the longest such closure in U.S. history ― until the president agreed to reopen the government for three weeks while Congress negotiated border security funding.

On Feb. 15, Trump signed Congress’ spending bill that allocated $1.375 billion for 55 miles of “pedestrian fencing” ― a fraction of his initial demand. He also declared a national emergency to reroute about $3.6 billion from Defense Department construction projects and another $2.5 billion the department had allocated for counter-drug activities to build the wall.

Amash was one of 13 Republicans in the Democratic-controlled House last week to vote in favor of a resolution to block Trump’s emergency declaration. The Republican-controlled Senate is to vote on the resolution next week.

Sen. Rand Paul (R-Ky.) on Sunday became the fourth Republican senator to announce his intentions to vote in favor of the resolution, increasing the likelihood that the measure will clear Congress and be sent to Trump’s desk. That would force a presidential veto, the first of Trump’s presidency.

Amash on Sunday said it’s possible a “national security crisis” exists on the southern border, but said the president shouldn’t simply bypass Congress to address it.

“The president doesn’t get to decide that he can override Congress simply because Congress doesn’t do what he wants,” Amash said. “If there were an emergency in the sense that the president is describing, there would be a lot more consensus.”

Asked whether Republicans who support Trump’s national emergency are “abdicating their responsibilities to the Constitution,” Amash said yes.

“I don’t think that they are all intending to do that,” he told CNN’s Jake Tapper. “But I think the president is violating our constitutional system. And I don’t think Congress can grant legislative powers to the president by statute.”

He added: “The best check on the president’s action is Congress. It’s not the courts. Our system is not designed so that the courts are going to resolve these disputes all the time between the legislative branch and the executive branch. We have to protect our own power. And that’s what I’m doing. And I’m hopeful many Republican senators will agree.”

Amash, who did not endorse Trump during the 2016 election, also said he’s not ruling out a third-party bid for president in 2020.

“I’d never rule anything out,” he said. “That’s not on my radar right now. But I think it is important that we have someone in [the White House] who’s presenting a vision for America that is different from what these two parties are presenting. Right now, we have a wild amount of partisan rhetoric on both sides.”