The Democratic primary is down to two major candidates: Sen. Bernie Sanders and former Vice President Joe Biden.

Trade policy stands out as a crucial difference between them in key swing states, including Michigan.

Sanders seems to think that bashing Biden on trade is his ticket to victory, recently saying: “Joe is going to have to explain to the people, to the union workers in the Midwest, why he supported disastrous trade agreements.”

That approach may have had some success in past elections, but the politics of trade have changed.

Polling shows Americans in general have become more pro‐trade in recent years, and Democrats in particular have leaned in that direction.

Sanders and Biden need a positive message on trade to respond to President Donald Trump’s criticism of the system and his bragging about his various revisions to it.

But as things stand now, both candidates lack a coherent position on trade policy and both need to figure one out in a hurry.

Sanders is in the more difficult position, as he has been extremely critical of trade and trade agreements over the years.

Like Trump, Sanders says he is worried about trade deficits and jobs being sent abroad, and he would like to see the U.S. employ more people in manufacturing.

He is particularly concerned about competition with producers in poor countries and outsourcing of jobs to these countries. However, when he had an opportunity to vote on a trade agreement with Australia, an industrialized country, his position did not change and he supported classic protection of domestic industries.

On the other hand, he has tried to distance himself from Trump on trade and to offer a bit more nuance. In 2018 he called the Trump administration’s “national security” tariffs on steel and aluminum “haphazard and reckless,” noting that “(imposing) tariffs on Canada and the European Union is an absolute disaster that will cause unnecessary economic pain to farmers, manufacturers and consumers in Vermont and throughout the country.”

What Sanders needs to do is move beyond vague criticism and lay out some details of his trade agenda.

Which of Trump’s tariffs would he retain and which would he withdraw? Is there any trade agreement Sanders would vote for? What specific changes would he make to U.S. trade agreements? Which countries would he prioritize in trade negotiations? And how would he approach trade relations with China?

Biden is in a different position because he was a supporter of the pre-Trump system. As vice president, he campaigned for the Trans-Pacific Partnership from which Trump eventually withdrew. Biden now says: “I would not rejoin the TPP as it was initially put forward. I would insist that we renegotiate pieces of that with the Pacific nations that we had in South America and North America, so that we could bring them together to hold China accountable.” As a political position, that is reasonable, but in terms of policy it leaves important questions open: What specific changes would he push for in a renegotiated TPP?

There are plenty of reasonable answers to that question, and Biden should show us that he has some in mind. Some of the questions for Sanders apply to Biden too: What would a Biden trade agreement look like? Which countries would he prioritize? And more specifically for Biden, what did he learn from the failed efforts of the Obama administration to get Congress to pass the TPP? Who will he put in charge of trade policy in his administration?

Perhaps the only benefit of Trump’s trade protectionism is that it has demonstrated the costs of tariffs.

American voters tend to be pro-trade these days, and a Democratic candidate should have a vision to offer them. We know what Trump’s vision looks like: disrupting existing trade arrangements and renegotiating them in a protectionist direction that makes them worse and aggravates our allies.

That offers an opportunity for Sanders and Biden to appeal to voters by setting out a coherent and positive message on trade.

Simon Lester is the associate director of the Cato Institute’s Herbert A. Stiefel Center for Trade Policy Studies. This article appeared in The Detroit News on March 9. Send comments to [email protected].