Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (R-Ariz.) on Monday issued a warning to 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 over his recent moves on trade, saying trade wars "are not won, only lost.”

“Free trade is not a zero-sum game. Mexico and Canada have benefitted handsomely from NAFTA — and so have we. Trade wars are not won, only lost,” Flake, a frequent critic of the president, wrote on Twitter.

Free trade is not a zero-sum game. Mexico and Canada have benefitted handsomely from NAFTA - and so have we. Trade wars are not won, only lost.https://t.co/cGmHvzYVT6 — Jeff Flake (@JeffFlake) March 5, 2018

The president last week announced plans to impose tariffs of 25 percent on steel imports and 10 percent on aluminum imports, a move that rattled Washington and the global markets.

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Trump also touted the possibility of a trade war, saying “trade wars are good, and easy to win.”

Trump then added a new wrinkle to the administration’s plans on Monday, writing on Twitter that the recent tariffs he announced “will only come off” if Mexico and Canada reach a “new & fair” North American Free Trade Agreement (NAFTA) with the U.S.

“We have large trade deficits with Mexico and Canada. NAFTA, which is under renegotiation right now, has been a bad deal for U.S.A. Massive relocation of companies & jobs,” Trump said.

The president frequently railed against trade deals like NAFTA on the campaign trail in 2016, arguing they hurt American workers.

In an interview that aired last week, Flake wouldn’t rule out a 2020 challenge to Trump and said he thinks the president “will have a challenge” from a member of the GOP.

“I do think the president will have a challenge from the Republican Party, I think there should be,” Flake said. “I also think that there will be an independent challenge, particularly if the Democrats insist on putting somebody up from the far left of the party.”