There's a trade war raging between the top two Democratic presidential contenders. Since Super Tuesday left two main candidates standing last week, Bernie Sanders has made it a point at campaign stops to list the areas where he and opponent Joe Biden disagree. Trade has taken center stage this week, as the two men fight for delegates in the industrial Midwest.

Though neither candidate has released a detailed trade policy, a decisive primary win in Michigan on Tuesday or next week in Ohio could signal how the Democratic Party will approach trade and the nation's economic agenda.

Across one of the nation's most iconic manufacturing states, Sanders has been taking swipes at Biden's positions on trade. He slammed then-Senator Biden's support of the 1993 North American Free Trade Agreement (NAFTA), which eliminated most tariffs between the U.S., Mexico and Canada, and the 2000 China Trade Bill, which reinstated permanent and normal trade relations (PNTR) with China. Sanders voted against both measures as a House member and continues to rail against the policies today, a point highlighted by a Sanders ad, "Decimated," that's up in every March 10th and March 17th primary state.

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"If we are going to defeat Trump in Michigan, in Pennsylvania, and in Wisconsin, it will be very hard for a candidate who voted for these disastrous trade agreements," Sanders told a Detroit crowd Friday.

This playbook may seem familiar to Democrats, since Sanders waged a similar campaign against Hillary Clinton in the 2016 Democratic primaries in Michigan. He skewered her at a Democratic debate held in Flint, Michigan, two days before the primary.

"Secretary Clinton supported virtually every one of the disastrous trade agreements written by corporate America. NAFTA, supported by the Secretary, cost us 800,000 jobs nationwide; tens of thousands of jobs in the Midwest. Permanent normal trade relations with China cost us millions of jobs," he said at the time.

The argument helped Sanders eke out a win over Clinton in the Great Lakes state, in a 1.5-point upset. Later, Donald Trump, too, consistently criticized Clinton over NAFTA and the Trans-Pacific Partnership.

Sanders portrays his congressional votes opposing the trade deals as a matter of character.

"I worked with the unions as a member of the House of Representative to defeat NAFTA and to defeat PNTR with China. I can remember like it was yesterday being on a picket line in Montpellier, Vermont in opposition to NAFTA. I knew what it was gonna do. I knew that you cannot ask American workers who are earning living wages to compete against the starvation wages being paid in Mexico," Sanders said Friday in Detroit.

The senator hit the same note during campaign stops in Michigan over weekend.

He said in Flint Saturday, "I heard, during that period in the '90s, I heard all of corporate America say, 'You gotta vote for these trade agreements.' And all of the media said, 'You gotta vote for these trade agreements.' I stood with the unions, the working families of this country; I voted against those agreements! Joe Biden voted for those agreements."

The reviews of NAFTA are somewhat mixed, and a new version of the deal, the U.S.-Mexico-Canada trade agreement, is expected to be implemented soon, after Canada ratifies it. In 2013, the liberal think tank Economic Policy Institute projected that NAFTA caused, "20 years of stagnant wages and the upward redistribution of income, wealth and political power." At the same time, Foreign Affairs estimates that 14 millions American jobs still depend on NAFTA. And a 2016 report from the U.S. International Trade Commission concluded the policy, "had essentially no effect on real wages in the United States of either skilled or unskilled workers."

Biden has rarely brought up North American trade policy on the stump since the Iowa caucuses, where he would emphasize the impacts of the agricultural trade war with China. Instead, he casts himself as an ally to workers and union members, even though its some workers have fallen victim to the shift of U.S. production to cheaper alternatives in Mexico, as General Motors did in March 2019, eliminating 1,700 hourly jobs in Ohio alone.

He stands by his NAFTA vote, arguing that "no, it wasn't" a mistake to vote in favor of the deal, he said last May in Iowa. He has labeled himself a "fair trader," rather than a "free trader."

"We should treat other countries in a way they treat us," Biden said, "Just deciding it's all about trade deficit balancing...we need to do more."

The most recent ripple in the trade debate has been the USMCA agreement, an idea Barack Obama pitched in the 2008 primary. Biden said in principle he supported the agreement due to the increased labor and environmental protections, issues that he told CBS News last summer he believed should have been more thoroughly addressed "from the beginning" of the NAFTA negotiation in the '90s.

Sanders unapologetically voted against USMCA, stating that protections did not go far enough.

On this matter Sanders stood alone. His one-time Democratic adversaries criticized him for being too purist in his pursuit. Elizabeth Warren, Sanders' fellow progressive in the race, promised to support the USMCA and bargain for a better deal if elected.

The distinction was noted by some in the Rust Belt.

Frank Hammer, a retired 32-year General Motors employee, participated in a panel discussion on trade with Sanders Friday in Detroit.

"You were the singular, sole [candidate] that said — consistent with your past positions — that you were opposed to [USMCA] and I think that speaks volumes for the stance that you have taken on behalf of working class people and especially auto workers," Hammer said. "And I'm so grateful. Thank you."

On the trail Biden has dismissed China's status as an economic powerhouse, pointing out basic challenges Chinese leaders face, like providing enough potable water for their people, and boasting about the U.S. dominance in the future. "The idea that China is going to eat our lunch, it's bizarre," he said in Iowa in late January, suggesting that rather than viewing China as an existential threat, the U.S. could help China with some of its problems. The U.S. "can compete with China and insist that they follow the rules," he proposed.

Unlike the Social Security ads the Biden campaign is running in Michigan, two Biden advisers tell CBS News after reviewing their internal polling they decided to not engage Sanders on the intra-party trade war.

"At this point I'm not sure voters are looking for a vote-by-vote analysis," one Biden adviser told CBS News, "They're looking for a nominee."

Others agree that focusing on trade—even in Michigan—may not be as beneficial as Sanders hopes.

"I think it's a miscalculation from Sanders," Justin Wolfers, professor of economics and public policy at the University of Michigan, told CBS News, citing a Gallup poll that shows 67% of self-described Democrats say that NAFTA has been beneficial for the United States.

Wolfers said that with the election of President Trump "everything is a little upside down" in terms of trade politics because the GOP incumbent has made a stronger pitch for protectionist voters than any Democratic nominee is likely to make.

"Relatively speaking, Democrats are now the pro-trade party," Wolfers added.

Biden has a strong argument to make in Michigan on manufacturing, though, since he had a leading role in helping to negotiate the Obama administration's 2009 auto bailout of two of the big U.S. automakers, General Motors and Chrysler.

"Just like the automobile industry came back, Detroit is going to come back," Biden said in 2014. "It's not only an important city but it's an iconic city. It symbolizes the manufacturing might of the United States of America."

According to an invitation obtained by CBS News, Biden is expected to attend a high-dollar fundraiser in Michigan this week, co-hosted by Cynthia and Edsel Ford, the great-grandson of Henry Ford.

Adam Brewster contributed reporting for this story.