As they head into the 2012 campaign, Democrats are changing their definition of what it means to be rich. Forget about families making $250,000 a year. Today, the party is only interested in millionaires.

In speeches across the country, President Obama has vigorously demanded that “millionaires and billionaires” pay “their fair share” in taxes. Last month, the White House said tax reform should ensure that billionaires such as Warren Buffett pay at least as much of their income to the Internal Revenue Service as middle-class taxpayers do.

And on Wednesday came clear evidence of this shift: Senate Democratic leaders scrapped Obama’s proposal to cover the cost of his jobs bill by raising taxes on income over $250,000 a year, the old Democratic standard for defining the wealthy. Instead, they are proposing a 5.6 percent surtax on annual income of more than $1 million.

Democrats say their new focus is intended to bolster support for Obama’s jobs package. But its more important purpose is to clarify the party’s economic agenda heading into next year’s election.

Democrats have long argued that, in addition to cutting government spending, lawmakers should ask people at the top of the income spectrum to pay more in taxes to help tame the national debt. But setting the dividing line at $250,000, as Obama did during the 2008 campaign, “fuzzies the picture,” said Sen. Charles E. Schumer (D-N.Y.), the leading architect of the surtax proposal. “There are lots of people who either make $250,000 or are close” to it, Schumer said, particularly small-business owners and dual-income couples living in high-cost urban areas.