Cuban appeared with Democratic presidential candidate Kamala Harris in Wisconsin on Thursday and is set to hold a town hall for her on Saturday in Phoenix before heading to Michigan.
He wasn't the only billionaire on the campaign trail: Tesla CEO Elon Musk was in Pennsylvania on Thursday mobilizing Trump voters ahead of Election Day on Nov. 5.
Trump's plan to impose fresh tariffs on up to 60% of goods from China in a bid to boost U.S. manufacturing is a "crazy notion" that will punish consumers who will end up paying the bill, Cuban said.
"This man has so little understanding of tariffs, he thinks that China pays for that. This is the same guy who also thought that Mexico would pay for the wall," Cuban said.
"Did Mexico pay for that wall?," Cuban asked the crowd, who responded, "no."
Trump has maintained that his trade policies - which call for pricey tariffs on goods not only from rivals such as China but allies such as the European Union - would revitalize American manufacturing and yield enough revenue to ease concerns about a ballooning deficit.
"To me, the most beautiful word in the world is 'tariff,'" Trump said on Tuesday in a sometimes-tense interview at the Economic Club of Chicago.
The Trump campaign did not immediately respond to requests for comment.
Cuban, an entrepreneur and star in the popular "Shark Tank" television series, brings tech-savvy pop culture appeal and business credibility that is especially appealing to young men, who have become a key demographic in a race where the gender gap has widened as males overall have shifted to Trump.
Cuban credited Trump with having a coherent trade and tariff policy when he was a political outsider in the 1990's and early 2000s, but said his current ideas are just "gibberish."
"Donald Trump is the Grinch that wants to steal your Christmas," he said. "The Grinch doesn't understand how tariffs work... The Grinch is the one that's going to be putting these small businesses out of business."
Unlike Musk, who has donated nearly $75 million to Trump-aligned groups, Cuban is not a political donor. Federal Election Commission records show a single, $1,000 donation to Congressman Zoe Lofgren, a California Democrat, in 2002 under Cuban’s name.
