UPDATE 1-Obama to propose "Buffett Tax" on millionaires

* Obama to propose "Buffett Tax" on millionaires

* Populist gesture designed to appeal to his base (Updates with more details, background)

By Alister Bull

WASHINGTON, Sept 17 (Reuters) - President Barack Obama, in a populist step designed to appeal to voters, will propose a "Buffett Tax" on people making more than $1 million a year as part of his deficit recommendations to Congress on Monday.

Such a proposal, among suggestions to a congressional supercommittee expected to seek up to $3 trillion in deficit savings over 10 years, would appeal to his Democratic base ahead of the 2012 election but likely not raise much in revenues.

White House Communications Director Dan Pfeiffer said in a tweet on Saturday the tax would act as "a kind of AMT" (Alternative Minimum Tax) aimed at ensuring millionaires pay at least as much tax as middle-class families.

The "Buffett Tax" refers to billionaire U.S. investor Warren Buffett, who wrote earlier this year that rich people like him often pay less in tax than those who work for them due to loopholes in the tax code, and can afford to pay more.

Obama will lay out his recommendations in White House Rose Garden remarks at 10.30 a.m. (1430 GMT) on Monday and is expected to urge steps to raise tax revenue as well as cuts in spending.

But Congress is at liberty to ignore his suggestions and Republicans, who control the U.S. House of Representatives, have said that they will not agree to tax hikes.

The supercommittee of six Democratic and six Republican lawmakers must find at least $1.2 trillion in deficit savings before the end of the year to avoid painful automatic cuts, and is mandated to seek savings of up to $1.5 trillion.

Those savings are on top of $917 billion in deficit reduction agreed to in an August deal to raise the U.S. debt limit and Obama wants it to go further.

He has separately urged the supercommittee to consider $467 billion in tax increases on top of that goal to pay for a jobs bill he unveiled earlier this month.

The Buffett Tax could help energize Obama's base by highlighting a feature of the U.S. tax code that allows the super rich to pay lower rates of tax than less wealthy Americans because the bulk of their income is capital gains, dividends and the 'carried interest' earnings of hedge fund managers.

That is taxed at 15 percent, compared with rates of 10 to 35 percent on straightforward income. (Reporting by Alister Bull; Editing by Peter Cooney)