WASHINGTON (Reuters) - At the request of the White House, Senate Democrats have pushed back the effective date of their proposed tax increase on millionaires until after next year's elections, a party aide said on Thursday.

In doing so, Democrats also increased the size of their proposed surcharge on the income of millionaires to 5.6 percent from 5 percent, the aide said. As revised, it would kick in on January 1, 2013, rather than January 1, 2012.

Senate Democratic Leader Harry Reid announced the tax proposal on Wednesday to fund President Barack Obama's $447 billion jobs program.

Reid said that his chamber will begin consideration next week of Obama's proposal, which Republicans seem certain to block. They denounce it as an political gimmick.

(Reporting by Thomas Ferraro; Editing by Sandra Maler)