President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE’s plan to levy steep tariffs on steel and aluminum imports has rattled global markets, irritated members of his own party and sent key U.S. allies searching for an equivalent response.

The announcement on Thursday came as a surprise to some in the White House, as a legal review on proposals sent to the president from the Commerce Department had yet to be completed.

Here are five takeaways on the most dramatic step yet by Trump on trade — a central issue in his successful presidential campaign.

Trump is picking a fight with Republicans

Trump and Republicans in Congress — with the exception of a handful of GOP lawmakers representing high-tax districts — were remarkably unified on December’s tax-cut package.

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The result was a new law that is the White House’s crowning legislative achievement — and one the GOP hopes will preserve its majorities this fall in the House and Senate.

The imposition of steep tariffs on steel and aluminum, in contrast, is intensely disliked by seemingly most of the GOP.

“Kooky 18th century protectionism will jack up prices on American families, and will prompt retaliation from other countries,” Sen. Ben Sasse Benjamin (Ben) Eric SasseChamber of Commerce endorses McSally for reelection Ben Sasse is mistaken with idea for the election of senators in America Big Ten football to return in October MORE (R-Neb.) said in a Friday statement. “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."

GOP leaders were nearly as critical, and Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE (R-Wis.) and other leaders have asked Trump to reconsider.

The party that is unified generally wins. On these tariffs, the GOP-led Congress and the White House is hopelessly divided.

Trump’s plan is dividing his own White House

There are deep divisions among Trump’s own team over the tariffs.

A cadre of senior officials, including National Economic Council Director Gary Cohn, Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE and national security adviser H.R. McMaster, oppose the tariffs and for more than a year had succeeded in holding them off.

Their success came, in part, because of a weekly trade meeting run by former top aide Rob Porter. The gathering was designed to ensure that officials on both sides of the debate had their voices heard, but also resulted in retaliatory trade actions being delayed.

That all changed when Porter resigned last month over domestic abuse allegations.

Trade hawks like Commerce Secretary Wilbur Ross Wilbur Louis RossTrump 'very happy' to allow TikTok to operate in US if security concerns resolved TikTok, WeChat to be banned Sunday from US app stores The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE, trade adviser Peter Navarro and U.S. Trade Representative Robert Lighthizer Robert (Bob) Emmet LighthizerWhiskey, workers and friends caught in the trade dispute crossfire GOP senator warns quick vote on new NAFTA would be 'huge mistake' Pelosi casts doubt on USMCA deal in 2019 MORE were given a freer hand to make their case to Trump, who has long wanted to impose tariffs on imported goods.

The president's decision to buck the free traders could trigger yet another staff shakeup. Cohn is said to be considering leaving the White House over the issue.

Ross and his allies aren’t showing signs of backing down. “I believe that everybody should rally round the president’s decision,” he said Friday.

U.S. allies may be hit the hardest

Canada, which is the largest importer of steel and aluminum imports into the United States and the biggest consumer of U.S. goods, may bear the brunt of Trump’s tariffs.

At 16 percent, Canada is the top source of imported steel into the United States, sending 90 percent of its steel here. Canada also holds the top spot for aluminum, accounting for more than 40 percent of all U.S. imports of the metal.

There is concern that the tariffs would significantly disrupt the intricate supply chain between the two countries.

Supporters of the steel and aluminum tariffs also argue that Canada isn't the problem.

Canada’s government, which isn't expected to get a hoped-for exemption, has already promised retaliation if Trump follows through.

Canadian Prime Minister Justin Trudeau called Trump’s plan “absolutely unacceptable” and credited the integration of North American steel and aluminum markets with creating millions of jobs.

The tariffs aren’t that much of a surprise

While Trump caught many people off guard with his announcement, his decision to impose tariffs really isn’t much of a surprise.

Trump ran on a promise to protect U.S. workers, arguing they had been betrayed by politicians in both parties who had negotiated away their jobs.

The steel and aluminum tariffs represent Trump’s effort to effectively fulfill a promise.

“The president has been consistent all the way from his campaign days to the present about doing something big to protect steel and aluminum,” Ross said.

“So it should not have come as a shock.”

Swing states may get the final say

Trump won the presidency because he captured victories in Pennsylvania, Wisconsin and Michigan, three states that a GOP presidential candidate hadn’t won in decades.

There are various reasons for his win, but his tough message on trade was clearly a factor in those states, as well as the perennial swing state of Ohio.

The two senators from that state, Republican Rob Portman Robert (Rob) Jones PortmanMcConnell locks down key GOP votes in Supreme Court fight Romney undecided on authorizing subpoenas for GOP Obama-era probes Congress needs to prioritize government digital service delivery MORE and Democrat Sherrod Brown Sherrod Campbell BrownBipartisan praise pours in after Ginsburg's death Emboldened Democrats haggle over 2021 agenda Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election MORE, notably both offered various degrees of support for Trump’s move, insisting that it could help producers in their states competing with China.

“I have argued that certain parts of our industry here do need immediate protection,” said Portman, who urged Trump to take a more targeted approach to the tariffs.

Foreign governments retaliating against the tariffs are likely to try to hurt these states. The European Union is already drawing up a possible retaliation list that includes motorcycles built by Harley-Davidson in Wisconsin.

How voters in these states ultimately judge Trump may be a deciding factor in what happens with the tariffs.

Keep an eye on Michigan, where automakers have an interest in steel tariffs.

Matt Blunt, president of the American Automotive Policy Council, said the tariffs “would place the U.S. automotive industry, which supports more than 7 million American jobs, at a competitive disadvantage.”