WASHINGTON — Senior citizens are knocking on senators' doors. Lobbyists for the homebuilding and energy industries are burning up their telephone lines.

Interest groups ranging from vulnerable people who tug at lawmakers' heartstrings to powerful contributors to their political campaigns have joined the push for $40 billion in add-ons to a House-passed economic stimulus bill.

The groups are leaning on wavering Republicans to support a $200 billion-plus economic aid package in the Senate, setting the stage for a politically vexing vote Thursday on the Democratic-written plan.

The economic stimulus bill that shot through the House in a burst of bipartisan agreement last week remained stalled in the Senate on Tuesday. Behind the scenes, the diverse coalition of lobbyists and grass-roots organizers seemed in perpetual motion.

Senior citizens were asking senators to support extending $500-$1,000 rebates to 20 million elderly people and 250,000 disabled veterans left out of the House plan.

Lobbyists for the homebuilding and energy industries were calling and e-mailing, eager to take advantage of lucrative tax breaks the measure provides for their sectors.

"Our lobbyists are up there almost on a 24-hour basis right now, doing everything we can in our power to try and get senators to support this," said Jerry Howard of the National Association of Home Builders. "Our guys are watching this very closely. We will be disappointed in folks who don't get behind it."

Homebuilders — they have given political candidates $4.2 million during this election cycle, according to the Center for Responsive Politics — have made more than 1,000 calls to twist arms on the plan, Howard said.

President Bush has warned the Senate not to turn the $161 billion House-passed aid plan into a Christmas tree loaded with special-interest items that would do little to stimulate the flagging economy. Up until Tuesday, Senate Republicans were joining Bush in demanding that the plan be kept clean in the interest of speeding its $600-$1,200 rebates to more than 100 million Americans.

In a mark of the political power of the seniors' lobby, though, Senate Republicans said Tuesday that they, too, would push to add rebates for the elderly and for disabled veterans to the House package.

Democrats want to tack on much more, including a $14.5 billion unemployment extension, $1 billion in heating assistance for the poor, and a tax break for businesses suffering losses that would cost $23 billion over the next two years. Their plan also contains more than $5 billion over the next 10 years in energy tax breaks.

A vote on that package could occur Wednesday evening or Thursday, and Democrats were toiling to collect the 60 votes they would need to advance it over GOP objections. They were counting on intense pressure from the outside groups to persuade wavering Republicans, including those facing tough re-election fights, to back the plan.

"That is a hard vote for a lot of Republicans, because they have sweetened it," said Sen. John Thune, R-S.D., who said he intends to oppose the package. He acknowledged that he had heard an earful from lobbyists from the wind power industry as well as other energy sector players who want him to go the other way.

Utility companies in states including Florida, South Carolina and Pennsylvania were contacting their senators to push for adoption of the plan. So were Realtors pressing for housing measures in the package, U.S. carmakers that have suffered losses and are hoping to take advantage of the tax breaks, and disabled veterans who would get rebates.

Senate Majority Leader Harry Reid, D-Nev., said Republicans would have to explain to the groups why they opposed the package.

"It's a burden they have. And with the elections coming in a short period of time, it's something they should focus on," Reid said.

Democrats are "trying to pick off individual Republicans," Thune said, although he added that Republicans are "pretty much hanging together" against the plan.

Senior citizens took to the hallways for a "lobby day" Tuesday to visit all 100 senators, urging them to back the measure.

"It's always a subliminal thing. If a group of seniors go in to see a senator, he has to recognize that this is a voting bloc," said Sue Ward, grass-roots director for the National Committee to Preserve Social Security and Medicare.

They weren't always successful. Confronted in his office by a senior confined to a scooter, Republican Sen. Jim Bunning, R-Ky., wouldn't commit to backing rebates for the elderly, Ward said. "He was kind of desperately trying to get away," she added.

Bunning opposed a similar measure when it passed the Finance Committee last week.

But the pressure clearly was building on some Republicans.

Sen. Norm Coleman said his Minnesota constituents were clamoring for the heating aid.

"I'm pretty inclined to support it," Coleman said

Sen. John E. Sununu of New Hampshire, another Republican facing a difficult re-election battle, noted that he had been a "consistent supporter" of the heating assistance, although he declined to say whether he would support the package, which he said he had not seen.

Sen. Ted Stevens, R-Alaska, who is also facing a tough race, was pushing for a separate vote that would add rebates for seniors and disabled veterans to the House-passed bill, and include another change to ensure that illegal immigrants could not receive checks.

Reid promised that Republicans would not get that chance.

And for now, that strategy has the backing of the most powerful seniors group, AARP, which has warned senators that it will keep track of their votes on the economic aid plan. AARP is supporting the broader package and pressuring lawmakers to do the same. Its members have made more 215,000 contacts, including calls and e-mails, to send that message, said CEO Bill Novelli.

"Obviously there are things in there that don't directly concern our folks, but we believe the best way to proceed is to support the full package," Novelli said.

He was on Capitol Hill releasing a poll showing that 90 percent of Americans support adding rebates for seniors to the stimulus bill. And he was accompanied by a roomful of elderly people clamoring for their checks.