Facing the prospect of defections from vulnerable lawmakers, Senate Republican leaders on Wednesday were busy whipping their Members to block Democrats from altering a fragile House-brokered economic stimulus bill.

That effort was complicated by the fact that Senate Finance ranking member Chuck Grassley (R-Iowa) joined Chairman Max Baucus (D-Mont.) in crafting their own stimulus plan, which the Finance panel passed on Wednesday. Grassley has pledged to help Baucus fend off other amendments that could make that package larger and more unwieldy.

But Senate Chief Deputy Minority Whip John Thune (R-S.D.) said GOP leaders’ objective in the coming floor debate will be to make sure the Finance stimulus package does not get a filibuster-proof 60 votes. He declined to detail how Republicans planned to do that.

Passing the Finance measure, Thune said, would send the bill to conference with the House, a scenario that would “wreck the balance that was struck” between the White House and House Democrats and Republicans.

Indeed, President Bush, along with Speaker Nancy Pelosi (D-Calif.) and House Minority Leader John Boehner (R-Ohio), warned the Senate repeatedly that any change to the House measure threatens to delay or derail the bill’s enactment.

And because Senate Democrats decided to create their own package, the path forward is fraught with peril for the bill.

Sen. Jim DeMint (R-S.C.) summed up the difficulty facing the Republicans, who do not want legislation that adds too much to the deficit, nor a process that would shut them out of offering amendments. “If we bring the Finance Committee bill to the floor and start adding to it, we’re going to be in a lot of trouble. But if they bring the Finance Committee bill to the floor and don’t let us amend it, they’ll be in trouble,” he said.

That’s why, DeMint and other Republicans said, the Senate should simply take up and pass the House measure without amendment.

But with 23 Senate seats to defend this year, Republicans have no shortage of vulnerable Members eager to put their mark on the stimulus bill or to vote for politically popular programs that were not included in the House-White House agreement, including tax rebate checks for more than 20 million seniors and 250,000 disabled veterans. And if Democrats largely stick together on votes adding provisions to the bill, they would need to pick off only nine to 12 GOP Senators, by most estimates.

“Unemployment in my state has gone from 4.4 percent to 4.9 percent,” lamented Sen. Norm Coleman (R-Minn.), who faces a tough re-election race this year.

“I haven’t seen the whole package,” he said, but “I would be open to adding a few targeted things to the package,” including unemployment aid, as the Finance package would do.

Similarly, Maine Republicans Susan Collins — who is also up for re-election — and Olympia Snowe have been seeking to add funds for low-income home heating assistance, a proposal Democrats are likely to offer on the Senate floor.

However, Sen. Judd Gregg (R-N.H.) said Senate leaders were attempting to make the case to wavering Members that a bipartisan deal — the House-passed measure, which he called the “Kumbaya package” — is already in hand and that anything else could cause needless delays.

“There is a lot of pressure in doing a stimulus quickly,” Gregg said. “There is a genuine desire to not do any more harm.”

However, the chance to pick off GOP moderates, along with Grassley’s support, forms the basis for much of Baucus’ continued optimism that his bill will garner a filibuster-proof majority on the floor.

Baucus did have to modify a key concession he made in his attempt to attract more Republican votes. Facing overwhelming opposition from Senate Democrats and a lack of traction among Republicans, he and Grassley reimposed income eligibility caps on the tax rebate checks. Baucus’ attempt to eliminate the caps, which came at the request of Grassley, had prompted a “gag reflex” from Democrats the day before, according to Senate Majority Leader Harry Reid (D-Nev.).

Democrats had balked at providing rebate checks to millionaires, but the new caps were still roughly double the House caps, starting to phase out at $150,000 in adjusted gross income for individuals and $300,000 for couples. To avoid a political backlash, Baucus’ measure also would prohibit Members of Congress from getting tax rebates, and he included a provision aimed at preventing illegal immigrants from getting the rebates.

Foreshadowing the upcoming debate on the floor, the Finance panel markup was filled with tension, as the majority of Republicans sought to point out Baucus’ folly in attempting to change a White House-brokered deal. In the end, only two committee Republicans — Snowe and Sen. Gordon Smith (Ore.) — joined Grassley in voting for the Baucus measure. Sen. John Sununu (N.H.), a politically vulnerable Republican, voted against Baucus’ package, explaining that he feared it would delay getting a deal.

Treasury Secretary Henry Paulson backed up Sununu’s contention Wednesday and indicated that many of the provisions sought by the Senate have already been rejected by the White House.

“The danger, which we need to avoid, is trying to make this too complex — add provisions to this — because so many of the ideas that are coming up right now from, again, people on both sides of the Senate, were the same thing” as what came up in negotiations with the House, Paulson said at a Real Estate Roundtable meeting.

Meanwhile, Reid acknowledged Wednesday that the Baucus plan and other amendments would likely have to overcome a 60-vote hurdle to be adopted. That may mean Reid and Senate Minority Leader Mitch McConnell (R-Ky.) will agree to artificially impose that limit to speed the process, rather than employ the usual time-consuming method of filing procedural motions on every proposal.

As of press time, it was unclear how Reid planned to bring the bill to the floor. Earlier Wednesday, one Senate Democratic aide said Democrats were considering using a parliamentary option known as “filling the amendment tree” that would effectively prevent Republicans from offering counterproposals. However, other Democratic aides held out the possibility that Reid and McConnell could agree to a finite list of amendments with time limits for debate.

Still, Reid on Wednesday appeared to be ratcheting up his defense of the Senate’s decision to come up with its own bill, suggesting that even a potential veto threat from President Bush may not deter the chamber from moving forward.

“What we do here we don’t have to get sign-off from him,” Reid said.