WASHINGTON — Senate Democrats will move to add to a $150 billion economic stimulus package rebates for senior citizens living off Social Security and an extension of unemployment benefits, setting up a clash with the White House and House leaders who are pushing a narrower package.

As the House planned a vote tomorrow on a plan that would speed rebates of up to $600 to most income earners — more for couples and families with children — the Senate was planning to draft its own measure with the add-ons, senior Senate aides in both parties said, speaking on condition of anonymity because the package is not yet final.

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.

Adding rebates for senior citizens living solely off Social Security checks — who are ineligible under the plan hatched by House leaders and the White House — would likely mean doling out smaller rebates overall, shrinking the size of the payments from $600 to $500, according to a senior Senate aide.

President Bush planned to use his State of the Union address tonight to call on Congress to move quickly on the agreement, the White House said.

Mr. Bush will tell “Congress, and specifically the Senate, not to delay or derail this agreement,” the White House spokeswoman, Dana Perino, said.

Still, pressure from the elderly and labor unions — both politically potent forces — is spurring senators from both parties to call for the extras.

The House plan leaves out some 20 million seniors, according to the AARP.

The Senate measure is likely to include a 13-week extension of unemployment benefits and a 26-week extension in states where the unemployment rate exceeds 6%, Senate aides knowledgeable about the package said.

The president and House leaders agreed last week on a proposal to provide rebates to 117 million families and to give businesses $50 billion in incentives to invest in new plants and equipment. The goal is to help head off a recession and boost consumer confidence.

But the Senate majority leader, Harry Reid, Democrat of Nevada, quickly said the Senate would consider adding to that plan, including extending unemployment benefits, boosting home heating subsidies, raising food stamp benefits, and approving money for public works projects.

The White House has warned against tinkering with the agreement reached with House leaders last week.

“I don’t think the Senate is going to want to derail this program,” Treasury Secretary Paulson, Mr. Bush’s pointman on the deal, told CNN yesterday. “And I don’t think the American people are going to be anything but impatient if we don’t enact this bipartisan agreement quickly.”