As the House planned a vote Tuesday 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, said senior Senate aides in both parties, 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.