And head of CNN's Democratic debate in Flint on Sunday night and Tuesday's primary, a full-fledged trade war has broken out.
Sanders, hoping to exploit what could be a major weakness for Clinton in labor-heavy states, called Clinton the "outsourcer-in-chief," latching Clinton to her husband's promotion of the North American Free Trade Agreement and her slow-to-the-game opposition to the Trans-Pacific Partnership. Clinton is hitting Sanders for siding with Republicans last year in a vote opposing the Export-Import Bank.
They're both playing to a labor-heavy audience, and both attacks get at the core of each candidate's critique of the other: that Clinton sways with the political winds, and that Sanders is too pie-in-the-sky to embrace the tools that deliver actual benefits to real people.
Clinton had dropped her attacks against Sanders in recent days after her dominant win in South Carolina and successful Super Tuesday, turning her attention to Donald Trump, the GOP and the general election. But she is now exchanging barbs with the Vermont senator and self-described democratic socialist.
In a speech Friday in Detroit, Clinton tagged Sanders as hewing to an "ideology" and "an old set of talking points," rather than offering a "credible strategy designed for the world we live in now."
One example: Sanders' vote to shutter the Export-Import Bank. The bank, which helps finance U.S. companies' exports into foreign markets and claims credit for supporting 109,000 jobs in 2015, has been a political flashpoint on Capitol Hill in recent years. Hardline conservatives -- including outside groups financed by the Koch brothers, who have labeled it a tool of crony capitalism -- have fought to shut it down, while Democrats and business-aligned Republican leaders have found legislative tools to keep it open.
Sanders was the only member of the Senate Democratic caucus to side with the GOP in a June 2015 vote. In a news release at the time, he labeled it "corporate welfare for multi-national corporations."
That's a controversial claim: Much of the bank's money helps Boeing, General Electric and Caterpillar. But the vast majority of the bank's customers are small businesses -- including 176 small businesses in Michigan since 2007, according to the bank.
"Unfortunately Republicans in Congress along with Sen. Sanders have tried to kill it repeatedly," Clinton said Friday in an economic policy speech at Detroit Manufacturing Systems in Detroit.
Sanders, as he has done much of the campaign, is painting Clinton as a flip-flopper who has only recently joined the left in opposing free trade deals like the 12-country Trans-Pacific Partnership.
Negotiated by President Barack Obama's administration, TPP is particularly controversial on the left. Sanders has opposed it for years. But Clinton waited until October to announce her opposition, saying she wanted to see the deal's final contents before taking a side.
"We have had for the last 25 years, disastrous trade policies. What these trade agreements are about, TPP, what these trade agreements are about," he said Friday in Edwardsville, Illinois. "They are written by corporate America and big money. They want to have an agreement that enables them to shut down plants."
Clinton's history, Sanders argues, is clear.
"The real question is was she right to support NAFTA? Was she right to support permanent normal trade relations with China? The answer is, she was very, very wrong and millions of families around this country have been suffering as a result of those disastrous trade agreements," Sanders said Thursday in Lansing, Michigan.
He added: "These trade agreements were pushed and written to a significant degree by corporate America for corporate America and the results are clear. We're not talking about some academic debate here. The results are clear when you look at Detroit, you look at Flint, you look at my own state of Vermont. We have lost thousands and thousands of decent-paying jobs."
The back-and-forth sets the frame for a broader debate over trade deals. Sanders is also assailing Clinton's vote for trade with China, her support for pacts with Colombia and South Korea (which cost U.S. steel jobs -- something Sanders is sure to highlight if the race lasts until Pennsylvania), and her delayed opposition to the Trans-Pacific Partnership.
Helping Sanders in his attacks are Republicans and the U.S. Chamber of Commerce, whose president, Tom Donohue, recently said he thinks Clinton would reverse herself and sign that pact if she were elected.
The Republican National Committee
on Saturday highlighted Clinton's past comments supportive of trade deals -- such as calling the TPP a "gold standard," the South Korea pact a "milestone" and the Colombia deal as one she opposed as a presidential candidate in 2008 but then was "absolutely committed" to implementing as America's top diplomat.
Friday, Clinton said that her approach is based on more than ideology.
"When it comes to trade deals, here's my standard: I won't support any agreement unless it helps create good jobs and higher wages for American workers and protects our national security," she said. "I need to be able to look into the eyes of any hard-working American anywhere in our country and say this deal will help raise your income."