The former Florida governor also doubled down on calling Trump a "jerk," knocking the businessman repeatedly during a swing across Iowa.
Bush mentioned Trump by name at least 17 times at his first two events, focusing on the Trump's plan to slap a 45% tariff on Chinese exports.
It's an idea -- aimed to crack down on trade imbalances with the country -- which Bush described as "really idiotic" and would have a tremendous impact in a state like Iowa, which trades heavily with China.
"Donald Trump doesn't know what he's talking about. And you know who'd be hurt the most? Iowa agriculture," Bush said at a town hall in Coralville. "What does a country do when it gets thrown a 45% tariff? It retaliates. And who would it retaliate with? Agriculture, and the equipment and the technology that this great state has developed and has been sold to China. All of that is foolhardy."
Later Tuesday, about 60 miles west in Grinnell, Iowa, Bush again ripped into Trump on trade.
"I'm, for one, not going to stand back when someone is advocating a global depression," he told reporters, arguing Trump's proposal would start a devastating trade war.
Bush finds himself far behind Trump in the polls, including in polls for the upcoming Iowa caucuses. A recent Quinnipiac survey showed Trump at 31% in Iowa with Cruz not far behind him at 29% and Bush at a mere 3%.
Despite Bush's low standing in the polls, Trump still enjoys grilling the former governor on the trail and on Twitter, keeping Bush at the top of his target list. Asked why he thinks Trump still cares about attacking Bush, the candidate reporters, "I don't care why he doesn't care or does care."
"I'm just telling you he's got unserious views on serious subjects," he added.
At that same event, which took place at Brownells firearm manufacturing plant, Bush was asked by a man in a wheelchair to weigh in on Trump mocking a reporter who has a disability -- something Trump denies doing.
Bush has been tearing into Trump recently over the incident, calling him a "jerk" unprompted late last month in New Hampshire. On Tuesday, he had no regrets about using the word to describe Trump, despite his insistence that a candidate can't "insult" his way to the presidency.
"I don't apologize for it because he is a jerk," Bush said. "He disparages POWs like John McCain, saying he's not a hero because he got caught. He's a jerk when he disparages women the way he does, and he is a jerk when he disparages people with disabilities. How are we going to win the presidency if we're tearing our country apart?"