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Days after quickly crafting a rare bipartisan compromise, the House on Tuesday overwhelmingly passed a $150 billion economic stimulus package to send checks to millions of low- and middle-income Americans.

But what shape the final legislation will take remains unclear as the Senate begins debate on its own plan.

Senate Finance Committee Chairman Max Baucus (D-Mont.) will convene a meeting Wednesday to consider his version, which differs substantially from the House bill. And other senators are pushing a range of proposals that would augment the deal worked out last week by House leaders and the Bush administration.

The House measure — a mix of tax rebates, business incentives and relief for strapped mortgage-holders — passed 385-35. But the lopsided vote did nothing to deter the Senate from seeking its own imprint on the legislation.

“What the House has done is important and good,” Senate Majority Leader Harry Reid (D-Nev.) said Tuesday, noting that there are 51 members of the Democratic caucus “without exception who believe this package can be made better.”

The majority leader said he has received 15 letters from lawmakers proposing additions.

The suggestions, which could jeopardize passage by mid-February, have set off alarm bells in the White House and House.

President Bush warned senators in his State of the Union speech not to “load up the bill.”

And Tuesday, House Speaker Nancy Pelosi (D-Calif.) repeatedly asked senators not to tie up the legislation. “I would hope that the timely aspect of this is recognized by our colleagues in the Senate,” she said.

The House measure relies primarily on individual tax rebates and temporary tax breaks to encourage businesses to create more jobs.

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So who gets how much, and when?

Q: Who would get a tax rebate under the House-passed bill?

A: A tax filer with at least $3,000 in income would get a rebate even if he or she didn’t pay income tax, but the rebate would phase out for the wealthiest taxpayers.

Q: How much would most taxpayers get?

A: Most individuals would get $600 and most couples would get $1,200 — plus all eligible people would get an additional $300 per dependent child.

Q: What’s the formula?

A: Individuals with an adjusted gross income of up to $75,000 and couples earning up to $150,000 would get rebates equal to the taxes they paid, up to $600 for individuals and $1,200 for couples. Those earning more than that would see their rebates go down by $50 for each $1,000 of income over the limits, meaning the wealthiest Americans would get no rebate. Taxpayers would get at least $300 even if they paid less than that in taxes — or $600 for couples. That’s also the case for those who don’t pay income taxes but earn at least $3,000.

Q: So when will we get our checks?

A: Not so fast. The plan goes to the Senate next, where prospects are uncertain. Some senators are seeking changes in the rebate, such as shrinking it to $500 for individuals and $1,000 for couples; sending checks to even the richest taxpayers, and including about 20 million senior citizens who are not due rebates under the House plan because they don’t have incomes.