Trucks line up to cross the border with the United States at Otay Mesa Commercial Port of Entry in Tijuana, Mexico on January 22, 2018. (Guillermo Arias/AFP/Getty Images)

Republican senators on Wednesday condemned President Trump's decision to impose tariffs on washing machines and solar panels, exposing simmering GOP divisions over international trade that threaten the uneasy alliance between the president and lawmakers of his own party.

"I don't agree with it, I think it's a bad path to head down," Sen. Roy Blunt (R-Mo.) said of the tariff decisions. "The retaliatory tariff fight is never a good fight and I generally think we need to be more positive about our trade opportunities."

The lawmakers said the tariffs could start a trade war that would damage the U.S. economy and threaten jobs, hurting the American workers Trump says he wants to help. The lawmakers also cautioned the administration to move carefully as it renegotiates the North American Free Trade Agreement — including during talks between U.S., Canadian and Mexican trade officials this week in Montreal. The White House is also considering whether to impose trade restrictions on imports of steel and aluminum, decisions that could have a widespread impact on the U.S. economy.

Sen. Mike Rounds (R-S.D.) said he was planning to sign onto a letter being circulated among Senate Republicans Wednesday expressing concern to the White House about the tariff decisions.

"I understand what the administration's trying to do — they're trying to send a message. And at the same time we want them to be very careful in terms of causing problems for organizations that are trying to do business in the United States who might still get hit with tariffs on their products," Rounds said.

The tariffs and uncertain nature of NAFTA discussions have stoked fears among GOP lawmakers that Trump will follow through on threats to withdraw from the trade pact. NAFTA enjoys widespread support among Republicans lawmakers, and before Trump, the party was largely united in favor of free trade.

GOP lawmakers have spent months trying to convince Trump to temper his "America First" trade approach, and they vowed on Wednesday to continue trying to persuade him to hold off on major changes.

But their campaign of persuasion faces the obstacle that Trump ran on a strictly protectionist platform, pledging to pull out of NAFTA and punish China and other U.S. competitors for unfair trade practices.

Trump believes that NAFTA and other past trade agreements have made it easier for other countries to sell their products in the United States in a way that destroys U.S. manufacturers through the use of cheap labor and low prices. This is a view shared by many Democrats, who have also said NAFTA should be renegotiated after more than 20 years.

Since taking office, he has not made good on most of his trade threats but many decisions still loom. The move on tariffs represented the most significant trade decision to date for the administration, and an apparent win for the protectionists in the White House.

In April, Trump came very close to beginning a process to terminate NAFTA, but he backed off following an intense lobbying campaign by lawmakers, business leaders, and some of his closest advisers.

He also came very close to starting a process that would have terminated a trade deal with South Korea, but tensions with North Korea intensified and he postponed that decisions as well.

Republicans are also treading carefully as they want to avoid getting crosswise with a president who's shown no compunction in going on the attack against lawmakers from either party when they go against his wishes.

"Well so what kind of a story are you writing? What a bad guy Trump is because he's doing tariff decisions? That's where everybody wants to go is to drag me into an article to be fighting with the president," said Sen. James E. Risch (R-Idaho) when questioned on the topic Wednesday. "Look, trade is really important. The president understands that. He wants a better deal for America, I support that. Details are always difficult to do. I don't want to work against the president, I want to work with the president to make trade work better."

Some Democrats are closer to the president on trade than members of his own party. Sen. Sherrod Brown (D-Ohio) applauded this week's tariffs and said he'd been dismayed when Trump delayed decisions about whether to impose restrictions on steel and aluminum imports.

Different groups of advisers within the White House have taken different approaches to trade policy.

U.S. Trade Representative Robert E. Lighthizer and Commerce Secretary Wilbur Ross have argued that the U.S. has been ripped off by other countries when it comes to trade policy, while White House National Economic Council Director Gary Cohn has urged more caution, warning that impulsive changes could harm the U.S. economy.

"The problem in part is you got the free traders whispering in one ear of the president, you got the people like Lighthizer and Ross whispering in the other ear, and the president seems too paralyzed on this," Brown said.

American manufacturers had complained for years that rising imports were hurting their sales. Trump imposed a tariff of 30 percent on solar panels in the first year, though it will recede to half that figure in four years. On washing machines, the first 1.2 million will face a 20 percent tariff, with additional imports facing a 50 percent levy. Imported parts for washing machines will also face a 50 percent tariff.

As he signed documents to kick off the tariffs, Trump again leveled a threat to withdraw from the North American Free Trade Agreement. His advisers have spent months trying to renegotiate terms of the deal with Canada and Mexico, though progress has been slow.

"NAFTA is moving along pretty well," Trump said Monday. "I happen to be of the opinion that if it doesn't work out, we'll terminate it … So we'll see how it all works out."

Opinions on NAFTA are well-defined among most GOP senators: They support it strongly, though some allow that there's room for improvement.

"I think withdrawing from NAFTA would be a disaster," said Sen. John Thune (R-S.D.). "We've made that clear."

The pushback comes as negotiators are meeting in Montreal in a sixth round of talks aimed at modernizing the 1994 treaty. Since getting underway in August, the three countries have made little headway toward overhauling an agreement that the president has repeatedly assailed as unfair to the American factory workers who helped him reach the White House.

Trump periodically has threatened to give notice of U.S. intent to quit NAFTA, which would trigger a six-month exit process. He has spoken of withdrawal as a possible negotiating strategy, though Mexico and Canada have vowed to abandon the talks rather than negotiate under duress.

As recently as January 18, Trump tweeted that NAFTA was a "bad joke" and suggested that Mexico would pay for a wall along the southern U.S. border through concessions in the talks.

At the same time, Trump has been hearing from farmers and industry groups that fear the loss of valuable export markets if he withdraws from the treaty. Up to 1.8 million U.S. workers would immediately lose their jobs if NAFTA were terminated, according to a new study funded by the Business Roundtable, a group of CEOs from companies such as JPMorgan Chase, General Motors and Walmart.

Negotiators are expected to make progress this week on parts of the treaty dealing with e-commerce and some — but not all of last year's gloom — has lifted. "I think we're in better shape," said one foreign official familiar with the talks.

With the tax legislation now behind him, the president has been eager to move forward on an aggressive bid to rewrite U.S. trade policy. With this week's steep tariffs on imports of solar panels and washing machines, Trump was exercising a presidential prerogative that had lain dormant since 2002, when President George W. Bush raised barriers to shipments of foreign steel.

That is widely expected to be just the beginning of the president's trade actions this year. Lighthizer, the U.S. trade representative, is preparing recommendations for U.S. action against China over its aggressive campaign to vacuum up U.S. trade secrets.

Citing national security, the administration also is mulling potential action against imports of steel and aluminum. Such measures might invite similar action by other countries that could certainly find or invent strategic rationales for protecting their businesses, analysts say.

David Lynch, Jeffrey Stein and Mike DeBonis contributed to this report.