President Donald Trump is “deadly serious” about a plan to put import tariffs on goods entering the U.S. from Mexico, his acting chief of staff said Sunday.

White House Chief of Staff Mick Mulvaney, in an interview on Fox News, defended Trump’s proposal to place an initial 5% levy on Mexican goods this month, rising as high as 25% by October if the nation doesn’t halt the flow of undocumented migrants to the U.S.

Mulvaney also rejected a suggestion that the tariffs endanger congressional approval of Trump’s new U.S.-Mexico-Canada trade pact, known as the USMCA—a replacement for the current North American Free Trade Agreement developed after over a year of intensive negotiations between the three countries.

“This is an immigration matter, it’s not a trade matter,” Mulvaney said, even as Trump boasted of potential job creation in the U.S. from the proposal.

Officials within the Trump administration—including Trade Representative Robert Lighthizer—opposed the tariff decision, worrying that it could poison the prospects of Trump’s proposed trade deal, according to people with knowledge of the matter. Some key congressional Republicans have also expressed their opposition.

‘A Mistake’

Republican Senator John Kennedy of Louisiana called the use of tariffs to curb immigration “a mistake” and on CBS’s “Face the Nation” said potential harm to the U.S. economy from steep tariffs would deter Trump.

“He’s a very smart man,” Kennedy said on Sunday. “He’s been known to play with fire, but not live hand grenades. And if he slaps a 25% tariff on Mexico it’s going to tank the American economy and I think the president knows that I don’t think he’ll do it. ”

Despite Mulvaney’s attempt to decouple the tariffs from being a “trade” issue, Trump this weekend touted the proposal as a way to boost job creation and move factories back to the U.S.

Markets Slump

“Our many companies and jobs that have been foolishly allowed to move South of the Border, will be brought back into the United States through taxation (Tariffs),” Trump tweeted on Sunday.

U.S. stocks and the Mexican peso slumped on Friday after Trump’s threat to impose escalating tariffs if no resolution is reached.

Asked how Trump would evaluate whether Mexico had satisfied his demands, Mulvaney said the actions Mexico would need to take were intentionally left “ad-hoc.”

Among the things the Mexican government could do, he said, were securing its own southern border with Central America, going after their “domestic terrorist organizations,” and making Mexico a safe place for people who wished to claim asylum.

“We need a vast reduction in the numbers crossing,” acting Homeland Security Secretary Kevin McAleenan said on CNN’s “State of the Union” about how he would judge if Mexico was responding adequately. “We need need Mexico to step up and do more.”