WASHINGTON (Reuters) - Taxes for America's highest earners have fallen sharply since 1995, according to a White House report on Tuesday, released ahead of a speech by President Barack Obama on fairness in the tax code that is a key part of his campaign for reelection.

The White House estimated the 400 highest income households in the country, who all earned over $110 million, paid an average of 18.1 percent of their income in federal taxes in 2007, well down for 29.9 percent those households paid in 1995.

Obama travels to Florida later on Tuesday where he will urge support for the Buffett Rule, named after billionaire investor Warren Buffett, that people making more than $1 million a year pay more in tax than middle class American families.

His populist message is also a clear swipe at Mitt Romney, the super-wealthy Republican likely to face Obama in the November 6 election, as the president seeks to appeal to blue collar voters that he will need to win a second term.

One of the richest men to ever seek the White House, Romney paid an effective tax rate of 13.9 percent in 2010 on more than $21 million in income. Obama's campaign is anxious to paint him as elitist and out of touch with ordinary Americans.

"Of millionaires in 2009, a full 22,000 households making more than $1 million annually paid less than 15 percent of their income in income taxes," the White House report said, citing date from the Internal Revenue Service.

Obama's push comes ahead of a Senate vote on the Buffett Rule on April 16. Republicans oppose the measure and say it won't create jobs or lower the high price of gasoline, which is worrying many Americans and could undermine the U.S. recovery.

"This is yet another sign that they're out of ideas and simply focused on tax hike show-votes," said Mitch McConnell, the top Senate Republican.

The seven page White House report was called "The Buffett Rule: A basic principle of tax fairness." It also noted that 1,470 households paid no federal income taxes on their million-plus-dollar incomes, and the rich were paying proportionally much less of their income than they had in the past.

(Reporting By Alister Bull; Editing by Cynthia Osterman)