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Warren Buffett isn't limiting his call for higher taxes to a minimum rate for very rich Americans who get a large chunk of their income from investments.

He's also one of several dozen wealthy people who have signed a statement calling for a "strong tax on the largest estates." It's been released by a group called "United For a Fair Economy."
Among the other signatories: Buffett's friend Bill Gates, former president Jimmy Carter, investor George Soros, and former Treasury Secretary Robert Rubin.

Noting that they have "benefited significantly" by government investments in schools, infrastructure. and public safety, among other things, the signer say it is "right morally and economically" to have a "significant" tax on large estates because it "promotes democracy by slowing the concentration of wealth and power."
"We believe it is right to have a significant tax on large estates when they are passed on to the next generation. We believe it is right morally and economically, and that an estate tax promotes democracy by slowing the concentration of wealth and power."

United For a Fair Economy Statement
The statement also notes that the government needs the money to both "fund vital services" and reduce the nation's budget deficit.

Under the current law, which expires at the end of the year, there is a $10 million exemption per couple. That "leaves too much revenue on the table," according to the statement.

(Read more: Buffett Expects 'Fiscal Cliff' Fix, but Not by December 31

Instead, Buffett and the other signers want a $4 million exemption, indexed to inflation, and a "graduated rate on the taxable estate over the exemption amount, beginning at 45 percent and rising on the largest fortunes."

The signers contend that only the top one percent of estates would have to pay taxes under their proposal.

(Read more: Warren Buffett's $250K Difference of Opinion with Obama)

This isn't a new cause for Buffett. Back in 2007, he went to Washington to appear before a Senate Finance Committee hearing on estate taxes. In his opening statement, Buffett said, "Dynastic wealth, the enemy of a meritocracy, is on the rise. Equality of opportunity has been on the decline. A progressive and meaningful estate tax is needed to curb the movement of a democracy toward a plutocracy."

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