WASHINGTON — In proposing a 5 percent surtax on incomes of more than $1 million a year to pay for job-creation measures sought by President Obama, Senate Democratic leaders on Wednesday escalated efforts to strike a more populist tone and to draw Republicans into a confrontation over how much affluent Americans should pay to help others cope with a struggling economy.

The White House, after dismissing a similar proposal late last year, left the door open to backing the plan. “We are open to different ways of paying for the very important broadly supported measures in the American Jobs Act that would grow the economy and create jobs,” said the press secretary, Jay Carney.

The new plan, devised by the Senate majority leader, Harry Reid, Democrat of Nevada, has a twofold purpose: to draw a sharp contrast with Congressional Republicans, who have dug in against any increases in tax rates, and to quell a revolt brewing among some Democrats who objected to parts of the White House plan.

Mr. Reid said the surtax would raise $445 billion over 10 years, just about the amount needed to pay for the jobs bill, though it appears unlikely it could make it through Congress.