As one of the few free-trade Democrats engaged in the public fray, I have a message for my former Democratic colleagues on Capitol Hill: You might not be backing trade, but our base has beat you to it.

Most Democratic candidates and officeholders still work under the assumption that Democratic voters oppose free trade. These politicians are spending too much time listening to labor-union lobbyists and antiglobalization activists. They should be spending more time reading opinion polls, which show that Democratic voters overwhelmingly support free trade.

In a survey earlier this year by the Pew Research Center, 67% of Democratic voters said free-trade agreements between the U.S. and other countries generally have been good for the U.S. Sixty-two percent of Democrats opposed tariffs, and 72% supported the North American Free Trade Agreement.

In another recent poll from Quinnipiac, 73% of Democrats opposed President Trump’s steel and aluminum tariffs, and in a poll by the Chicago Council on Global Affairs, 74% of Democrats described globalization and greater international economic ties as mostly good for the U.S. In the Pew poll, 60% of black and Hispanic voters—key Democratic constituencies—said they believe free-trade agreements have benefited the U.S.

To some extent, these results may reflect a visceral reaction against the president. Mr. Trump opposes trade, so some Democratic voters may have turned toward it reflexively. Yet Democratic voters are also responding to the increasingly evident economic harm caused by Mr. Trump’s antitrade policies. Ironically, his trade restrictions are showing many voters for the first time how much they benefit from trade.

What’s more, the damage caused by the trade war is being felt most deeply in areas where Democrats are prevalent. This may be counterintuitive, but the party Democratic leaders and activists see is not what our party has become two decades into the 21st century. For the most part, Democratic voters are not aggrieved workers in smokestack industries who blame foreign trade for their economic woes. Nor are most Democrats ardent antiglobalists wary of the growing interdependence and the encroaching economic integration of the world. Rather, Democrats live mostly in technologically advanced and digitally engaged metropolitan areas and are eager, productive participants in the global economy who benefit from globalization. One example is my hometown, Orlando, Fla., a new Democratic stronghold.

Though Hillary Clinton won only one-sixth of all U.S. counties in 2016, those she did win are extensively engaged in trade, accounting for 60% of U.S. exports. Democratic counties are especially overrepresented in services, which comprise 75% of gross domestic product and dominate international markets.

When I was in Congress in the 1990s, 100 or so House Democrats voted in favor of proposed trade agreements. Today perhaps a dozen or so can be counted on to do so. Some Democratic leaders still insist they are “for free trade, but . . .” Even so, many more congressional Democrats have spoken in favor of protectionism than against it.

As Democrats prepare to regain control of the House in January, they understand that most legislation they propose will be hindered by a Republican Senate and president. Yet on trade as on other issues, it is vital that Democrats let voters know where they stand before the 2020 election.

As part of a winning national strategy, Democrats in both houses of Congress should offer America a pro-trade agenda. Propose limitations to the president’s unilateral authority to restrict trade. Disavow trade actions that violate international agreements and stand up for continued impartial rulings by World Trade Organization judges. Vote to repeal Mr. Trump’s steel and aluminum tariffs, and to repeal auto tariffs if he imposes them. Insist that the “new Nafta” remain a free-trade agreement and not become a tool for managed trade. Publicly support a free-trade deal with Europe. Call for the U.S. to join the Trans-Pacific Partnership. Help workers dislocated by trade and automation build new skills to remain competitive in today’s job market.

By all means, Democrats must insist that China comply with all WTO rules. Urge the president to bring more WTO cases against China in areas where they violate the current rules, such as stealing trade secrets, illegally subsiding key industries, and forcing transfers of intellectual-property. Encourage Mr. Trump to work with trade partners to reform WTO rules on IP. But in the meantime, insist that the Trump administration stop challenging China by raising tariffs, which often violate international trade rules themselves.

Democratic leaders should take advantage of President Trump’s newly announced “trade truce” with China to seek solutions with America’s largest trading partner within the bounds of civility and WTO rules. And remember that a trade agreement with China that manages trade instead of freeing it would not be a solution.

If Democrats want a winning national strategy that will appeal to the rapidly growing segment of voters in the vibrant metropolitan centers, then we must include free trade in our governing philosophy. We must embrace an open economy as an essential part of an open society.