Would Republicans in Congress stay mute if a President imposed income or sales taxes on U.S. industries on an arbitrary whim? We doubt it, so it’s dispiriting to see Senate Republicans let Donald Trump impose tens of billions of dollars in border taxes without so much as a vote of protest.

That’s the sad story as GOP Senators last week blocked a vote on Bob Corker’s amendment to reclaim at least some of the power to impose tariffs that Congress has ceded to Presidents. Perhaps Mr. Trump took the silence as assent because he is escalating. On Monday he threatened tariffs on up to $450 billion in Chinese goods, and financial markets are finally losing their foolish complacency. Shares in exporters vulnerable to retaliation like Boeing and Caterpillar fell more than 3.6% Tuesday.

Mr. Corker’s bipartisan measure would have required Congress to approve trade restrictions that Mr. Trump is imposing under Section 232 of the Trade Expansion Act of 1962. This is the law that lets a President impose more or less whatever tariffs he wants with an elastic definition of national security. Mr. Trump has used this open-ended authority to inflict his 25% tariff on steel and 10% on aluminum, and he’s threatening a 25% levy on imported cars under the same law. His new China tariffs are based on a different legal rationale (Section 301).

“I would bet that 95 percent of the people on this side of the aisle support intellectually this amendment,” Mr. Corker said on the floor with some acidity. “And a lot of them would vote for it if it came to a vote. But, no, no, no. ‘Gosh, we might poke the bear’ is the language I’ve been hearing in the hallways.”

Mr. Corker is right that GOP leaders fear a Trump tweet in the middle of election season. Some of them are also griping in private that Mr. Corker has the luxury of bucking the President because he isn’t running for re-election. But Mr. Corker’s modest bill isn’t the political threat to Republicans. The growing damage from Mr. Trump’s trade war is.

By not allowing trade votes, Republicans are giving Mr. Trump free rein to impose tariffs that are doing substantial economic harm to many of their constituents. Farm state Senators deserve a chance to vote against tariffs that are spurring retaliation against U.S. agricultural exports of everything from pork to apples. So do Senators who represent U.S. manufacturers. The fear of a Trump tantrum is precluding an important fight about what the party of free enterprise supposedly believes.

The economic fallout may also hurt the GOP’s chances of holding the Senate in November. Democrats Heidi Heitkamp (North Dakota) and Claire McCaskill (Missouri) are running against the tariffs as a way to oppose Mr. Trump and defend their states’ agricultural interests. The longer Republicans shrink from standing up to Mr. Trump’s protectionism, the more voters will conclude that Republicans in Congress are complicit in the damage.

—The Wall Street Journal. June 19, 2018