The so-called "Fair Share Surcharge" would be imposed on the two out of every 10,000 taxpayers who make more than $5 million a year and is expected to raise $150 billion over a decade, the aide said.

Clinton was joined on the campaign trail last month by billionaire investor Warren Buffett, who has been an outspoken critic of tax policies he says allow him to pay a lower tax rate than his secretary.

Clinton said she would announce her plans to go even farther than the "Buffett rule," which would establish a minimum tax rate of 30 percent on those earning more than $1 million per year. The income-tax surcharge is the first part of her push, with more tax policies to be released later this week, a campaign aide said.

Clinton is the front-runner for the Democratic presidential nomination ahead of the election in November 2016. Both she and her chief challenger for the nomination, U.S. Senator Bernie Sanders of Vermont, have made income inequality and boosting the middle class a focus of their campaigns.