Hillary Clinton says presumptive Republican nominee Donald Trump would be "reckless" with the US economy and send the country into a recession.
His policies would hurt workers still recovering from the US recession in 2008, she said in Ohio.
But Mr Trump hit back by attacking her economic competence and insisting that he would "fix" the country.
The businessman and the former secretary of state are set to do battle for the White House in November.
Earlier this month, she cut down Mr Trump's foreign policy and said electing him would be a "historic mistake".
"Every day we see how reckless and careless Trump is," she said at a campaign event in Columbus. "Well that's his choice. Except when he's asking to be our president. Then it's our choice."
She said he does not have the temperament to lead the US economy and said small businesses and working families have reason to worry if he wins.
Mr Trump responded to her speech by referring to the FBI investigation into her use of personal email while at the State Department.
He said in a tweet: "How can Hillary run the economy when she can't even send emails without putting an entire nation at risk?"
He also tweeted: "I am 'the king of debt.' That has been great for me as a businessman, but is bad for the country. I made a fortune off of debt, will fix US".
Global markets "rise and fall" based by comments from the US president, Mrs Clinton said, and saying the US could default on its debts could cause a "global panic", she said.
She read the names of multiple Trump businesses that have failed in the past, on the heels of her campaign's new website and videothat detail the businessman's defunct ventures.
Mr Trump has criticised Mrs Clinton's support of the North America Free Trade Agreement, which he says has cost the US "millions of jobs".
Also this week, figures show that Mrs Clinton has a huge advantage in campaign funds over Mr Trump.
Mrs Clinton's campaign began June with $42m (£28m) in the bank; for Mr Trump it was $1.3m.
The businessman has claimed to have self-funded his campaign, portraying himself as an outsider not linked to special interests.
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