WASHINGTON—A lone senator jumped to the Trump administration’s defense last month when the U.S. Chamber of Commerce attacked its proposal to rewrite the North American Free Trade Agreement.

He is a Democrat.

“Any trade proposal that makes multinational corporations nervous,” says Ohio Sen. Sherrod Brown, “is a good sign that it’s moving in the right direction for workers.”

The politics of trade are shifting in Washington. In the 1990s, a Democrat in the White House, Bill Clinton, worked with free-market Republicans in Congress to open the world trading system to Mexico and China. Now a White House Republican, Donald Trump, hopes to work with trade skeptics among the Democrats to reverse that trade liberalization. It is too soon to say whether Mr. Trump will succeed. But if he does, it will be in alliance with people like Mr. Brown, a leading voice among Democrats in the Senate against free trade agreements written in the past three decades.

Already Mr. Trump has halted the steady liberalization of trade that began with President Franklin Roosevelt and continued in Democratic and Republican administrations. In his first working day in office, he pulled the U.S. out of a 12-nation Pacific trade pact, even though the other nations continue working to complete a deal. In a recent speech in Vietnam he warned Asian powers that the U.S. wouldn’t “be taken advantage of anymore” in trade deals. If anything, Democrats criticize Mr. Trump for not clamping the brakes even harder on globalization.