Republicans and conservative groups have come out strong against President Donald Trump's proposed new tariffs.

GOP Sen. Ben Sasse said the tariffs "will kill American jobs," while House Speaker Paul Ryan's office sent out an email Monday tying early stock market declines to tariff concerns.

Club for Growth, a conservative political action group that favors free markets, called the decision "an affront to economic freedom."

In a dramatic split from their party's leader, Republicans and conservative groups have begun targeting with scorched-earth potency President Donald Trump's decision to introduce new tariffs, arguing that they would be a disaster for the US economy and American workers.

AshLee Strong, a spokeswoman for House Speaker Paul Ryan, said Monday that Ryan was asking Trump not to go forward with the new trade measures.

"We are extremely worried about the consequences of a trade war and are urging the White House to not advance with this plan," Strong said. "The new tax-reform law has boosted the economy, and we certainly don't want to jeopardize those gains."

Her statement came after Ryan's office circulated a CNBC article attributing the decline in the stock market early Monday morning to tariff concerns.

Trump, in an Oval Office meeting with Israeli Prime Minister Benjamin Netanyahu, said Monday that he wouldn't back down on his proposals.

"No, we're not backing down," Trump said. "We had a very bad deal with Mexico — we had a very bad deal with NAFTA."

But Ryan's concerns were shared by other Republican lawmakers, who said the tariffs were a threat to the US economy.

"Kooky 18th-century protectionism will jack up prices on American families — and will prompt retaliation from other countries," GOP Sen. Ben Sasse said Friday. "Make no mistake: If the president goes through with this, it will kill American jobs — that's what every trade war ultimately does. So much losing."

Trump announced Thursday that the US would soon impose new tariffs, which act as a tax on imports, of 25% on steel and 10% on aluminum. Trump's trade advisers, like Peter Navarro, the director of the White House trade council, have said there will be no exemptions, though Trump suggested on Twitter on Monday morning that Canada and Mexico could receive some as part of ongoing trade negotiations.

Trump's moves have sparked concerns of a trade war in which countries retaliate with trade restrictions against the US. Already the European Union, China, and Canada have said they are considering reciprocal measures. Trump on Friday embraced that prospect, tweeting that "trade wars are good." He took a more measured tone in the Oval Office on Monday, saying that he didn't think there would be a trade war.

According to a GOP source, Congress could consider legislative action related to the new tariffs if Trump follows through.

Republican Sen. Lindsey Graham on Sunday attempted through an appearance on CBS' "Face the Nation" to make the president reconsider.

"On trade, you correctly identified the problem of China dumping steel throughout the world destroy the American steel industry," Graham said, addressing Trump directly. "Your solution is let China off the hook. It's only going to hurt American consumers and our allies. Please reconsider your solution."

Conservative political groups have also taken aim at Trump's decision. Club for Growth, a conservative group that favors free markets, said the tariffs would be bad for American workers in a broad swath of industries.

"The idea of imposing steel or aluminum tariffs of any kind is an affront to economic freedom," David McIntosh, the group's president, said in a statement. "First and foremost, it's bad for the American worker. For every steelworker job that might be saved because of a tariff, our country will lose even more American jobs in auto plants, construction, and so many other industries."