Hillary Clinton leaned into her plans to raise taxes on the wealthiest Americans on Wednesday, denouncing Donald J. Trump’s tax proposals as a boondoggle for billionaires.

“We’re going to tax the wealthy who have made all of the income gains in the last 15 years,” Mrs. Clinton told a crowd in Cleveland. “The superwealthy, corporations, Wall Street,” she declared emphatically, “they’re going to have to invest in education, in skills training, in infrastructure.”

For months, Mrs. Clinton has attacked Mr. Trump’s economic agenda in broad terms, portraying him as a follower of the “trickle down” orthodoxy of previous Republican administrations. But Mr. Trump’s release of his tax plans last week in Detroit allowed her to begin to criticize them more specifically.

Just as President Obama attacked his 2012 rival, Mitt Romney, for paying a lower effective tax rate than the vast majority of Americans, Mrs. Clinton said that Mr. Trump’s plan would benefit people in his own income bracket, declaring that he “would pay a lower rate than middle-class families” if it were put into effect. Mr. Trump has recommended cutting the top marginal income tax rate to 33 percent from the current 39.6 percent, and broadening deductions for things like child care.