Senate Democrats are banking on both a cadre of outside groups — from senior citizens to homebuilders — and election-year pressures to build momentum among Republicans for expanding a House-passed economic stimulus bill this week.

“It does appear that reports of the demise of the [Senate] Finance Committee version of the stimulus are greatly exaggerated,” said one senior Senate Democratic aide.

Early last week, Senate Democrats seemed to be attempting in vain to rework a deal supported by House Democrats, House and Senate Republicans, and President Bush. But by Friday, cracks seemed to appear in the Senate GOP leadership’s resolve to hold the line against any and all changes to the House-passed measure.

“This is about as fluid as anything I’ve seen,” said one knowledgeable Senate GOP aide.

Jim Manley, spokesman for Senate Majority Leader Harry Reid (D-Nev.), said Democrats were getting key support from homebuilders associations, veterans groups, and the largest senior citizen organization in the country, AARP.

“Groups are lobbying hard, working through the weekend,” Manley said. “And there’s plenty of time next week to build coalitions.”

Indeed, Senate Democrats are hoping the bill passed out of the Finance Committee last week contains the magic formula of focusing in on rebate checks for low-income senior citizens and disabled veterans as well as expanded small-business tax breaks.

AARP spokesman Jim Dau said the organization already directed more than 100,000 e-mails and phone calls to Senate offices from Wednesday to Friday morning last week, with more expected this week.

“We’re not taking anybody for granted, and we’re not leaving anybody off the target list,” Dau said of AARP’s efforts to secure passage of the Finance measure.

While AARP may not be cherry-picking its targets, Democrats are.

Aides said Democrats were courting Republicans with tough re-election bids as well as those who represent businesses that might benefit from the tax breaks included in the Finance measure. Those Senators include James Inhofe (R-Okla.), Ted Stevens (R-Alaska), Lisa Murkowski (R-Alaska), Richard Burr (R-N.C.), Elizabeth Dole (R-N.C.), Susan Collins (R-Maine) and Norm Coleman (R-Minn.).

Though three Republicans — Sens. Chuck Grassley (Iowa), Olympia Snowe (Maine), and Gordon Smith (Ore.) — already have announced their support for the Finance package, Senate Republican leaders are still expected to apply heavy pressure to the rank and file to oppose it, in part because it would cost nearly $11 billion more than the House-passed measure.

But it appeared increasingly likely at the end of last week that enough Republicans might vote with Democrats at least to make sure seniors and veterans received tax rebate checks.

“There may be some room for” seniors and veterans, the knowledgeable Senate GOP aide said, adding, “It’s really delicate in terms of making changes.”

The aide warned that Republican Senators would likely want some assurances from House leaders that if the Senate votes to include seniors and veterans, the House would not tinker any further with the bill.

“There’s a ton of caution to opening this up in any fashion,” the aide said.

If the Senate Finance plan fails to garner the 60 votes needed to beat back a filibuster, Democrats plan to hold votes on three other amendments, including one to add seniors and veterans to the House rebate formula.

Democratic aides said the other two amendments were in flux as of Friday, but previously, Reid said one would provide additional low-income home heating assistance funds and the other would be a package of Democratic priorities such as unemployment insurance and food stamps. Because of Friday’s Labor Department report showing a shrinking job market, Democrats may decide to have a stand-alone vote on extending unemployment benefits, one Democratic aide said.

Senate Democratic aides indicated that the seniors and veterans amendment might also include funding for low-income home heating assistance and a provision clarifying that illegal immigrants are not eligible for rebate checks. The House measure would provide limited small-business tax breaks and rebate checks to low- and middle-income workers, but not those whose earnings are derived from Social Security or veterans’ disability payments.

Because of a likely filibuster, Reid has acknowledged that most amendments will have to receive 60 or more votes for passage.

Of course, one of the keys to getting 60 votes will be the attendance of Democratic presidential contenders Sens. Hillary Rodham Clinton (N.Y.) and Barack Obama (Ill.), who are furiously campaigning for votes in the Super Tuesday primaries and caucuses. But Democratic aides said Clinton and Obama probably will not be needed for today’s scheduled vote on a procedural matter.

Even without the primary schedule, Reid might have trouble trying to hold any key votes on amendments before Wednesday, because he does not yet have an agreement with Senate Minority Leader Mitch McConnell (R-Ky.) that would allow the chamber to avoid jumping through some time-consuming procedural hoops.

A deal could be forthcoming today, but Republicans remain suspicious that Democrats are scheduling these votes only to provide the Democratic Senatorial Campaign Committee with fodder for election ads this year.

“The lesson from 2007 was that to get things done, you need to put the campaign aside and get to work on serious legislation. You’d think they’d learn that there’s a difference between the Senate floor and the boiler room at the DSCC,” said McConnell spokesman Don Stewart. “You would think that after the overwhelming, bipartisan support for timely economic relief by Speaker Pelosi and Leader Boehner, Senate Democrats would get the message. It appears they have not.”

Democrats did not deny that the votes might come up again on the campaign trail this year, but they insisted that they were not designed for political purposes

“The votes that are being planned are for the purpose of improving the stimulus, not to set up election-year issues,” said one senior Senate Democratic aide. “But it’s certainly true that Senators who oppose these measures … will have to account for that later.”

House Republicans, meanwhile, were eyeing the Senate machinations and hoping that the stimulus package doesn’t swell into an even bigger behemoth.

“House Republicans are waiting to see if the Senate will exercise some restraint and pass a stimulus package that is in fact a stimulus package,” said a GOP aide. “The more this legislation becomes weighed down with extraneous spending, the less it serves to actually give our economy the boost it needs.”

Steven T. Dennis contributed to this report.