House Speaker Paul Ryan signaled opposition to President Donald Trump’s proposed tariffs on steel and aluminum.

“We are extremely worried about the consequences of a trade war and are urging the White House to not advance with this plan,” Ryan’s National Press Secretary Ashlee Strong said in a statement. “The new tax reform law has boosted the economy and we certainly don’t want to jeopardize those gains.”

Two members of Speaker Paul Ryan’s communications staff emailed a CNBC article about the negative effects that Trump’s proposed tariffs were having on the stock market.

“FYI” wrote Strong in the subject line on an email, including the full text of the article that noted, “Stocks opened lower on Monday as investors mulled over potential U.S. steel and aluminum tariffs.”

Ryan has worked closely with the Trump administration for most of his presidency, only occasionally signaling dissent with the president’s tone. This is the Speaker’s first fundamental opposition to Trump’s protectionist economic policies that helped him win the 2016 election.