WASHINGTON — A top Senate Democrat unveiled a $156 billion economic stimulus package yesterday that awards rebates to senior citizens living off Social Security and extends unemployment benefits, setting up a clash with the White House and House leaders pushing a narrower package. As the House planned a vote today on a plan that would speed rebates of up to $600 to most income earners — more for couples and families with children — the chairman of the Senate Finance Committee, Max Baucus, scheduled a Wednesday vote on a bill with the add-ons, which would send rebates even to the wealthiest taxpayers.

“My proposal will give America’s seniors the same rebate as any wage earner,” Mr. Baucus said, a Democrat from Montana. “The White House says we mustn’t slow the economic stimulus agreement down, or blow it up. I agree. We’re going to improve it and get it passed right away.”

The move was in defiance of admonitions from the Bush administration not to risk derailing the deal with changes, and it threatened to slow what was shaping up as an extraordinarily rapid trip through Congress for the stimulus measure. The Senate hopes to pass its version by week’s end.

“I strongly support Chairman Baucus’s efforts to garner bipartisan support to improve the House agreement, and I intend to take legislation to the floor as quickly as possible to strengthen the economy,” the majority leader of the Senate, Harry Reid, a Democrat from Nevada, said.