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Mr. Romney has released his 2010 tax return and an estimate of his 2011 taxes, showing he paid a rate of 13.9 percent on income of $21.6 million in 2010 and 15.4 percent on income of $20.9 million in 2011.

Mr. Obama released his 2011 tax return last week, showing income just beneath the level that could have qualified for the Buffett rule tax. The president paid 20.5 percent of his income in federal taxes.

While administration officials blamed “tax breaks,” the chief target of the Buffett rule would be those who make money from investments. Under the 2003 Bush tax cuts, later extended by Mr. Obama, both dividend and interest income are taxed at 15 percent — well below the top income tax rates.

Republicans said it still amounts to double taxation at 15 percent because that income was already taxed at the corporate tax rate of 35 percent on the business side, and then is taxed again when disbursed to investors.

The Buffett tax would apply to about 223,000 taxpayers.

If imposed, the tax could save $4.6 billion to $16.2 billion in an average year over the next decade. Even at that high range, however, it’s still just 2.5 percent of the deficits Mr. Obama’s budget plans would produce during that 10-year period.

Democrats said every little bit can help.

Mr. Schumer vowed to try to use the Buffett tax as a way to pay for expanded college access or a research-and-development tax credit. He predicted that the GOP will have a hard time voting against the tax increase when the revenue raised is used for one of those purposes.