Democratic presidential nominee Hillary Clinton proposed on Thursday to tax the estates of ultra-rich Americans at a rate as high as 65 percent — a plan that would apply to only a handful of billionaire families, and which comes straight from the campaign playbook of Clinton's former rival, Sen. Bernie Sanders of Vermont.

Clinton had already proposed to raise estate tax rates on some millionaires to 45 percent. Her new plan goes further. It would add three new brackets: a 50 percent rate for couples with estates valued above $10 million, a 55 percent rate for couples with estates above $50 million and a 65 percent rate for those with estates above $1 billion. Republican nominee Donald Trump has called for the elimination of the estate tax entirely.

Internal Revenue Service data suggest Clinton's highest rates would apply to very few Americans. In 2014, there were only 223 estate-tax payers with reported estates valued at $50 million or more. Still, the plan drew immediate protests from conservatives, including the Trump campaign, which labeled it an "even more dramatic hike in the death tax."

Politically, the proposal could serve a dual purpose for Clinton. It would raise more revenue for the federal government and helps her make the case that she has proposed sufficient tax increases to offset all her proposed spending programs. It also could help her court former backers of Sanders, particularly young liberals who have been slower to embrace her candidacy after she defeated him for the nomination.

Sanders's spokesman Mike Briggs said the senator's economic policy aides have been talking to Clinton's campaign about this and other ideas for months and were aware that her plan was being released. Briggs described the Clinton proposal as identical to one that Sanders proposed  last year.

In a statement Thursday, Sanders said, "Secretary Clinton understands that it is appropriate to ask the top three-tenths of 1 percent, the very wealthiest people in this country, to pay their fair share of taxes so we can provide a Child Tax Credit for millions of working families and lower taxes for small businesses. Under this proposal, 99.7 percent of Americans would not pay a nickel more in taxes."

John Wagner contributed to this report.