President Bush worked the phones Monday night to influence lawmakers preparing to vote on a new homeland security agency as Democrats fought Republican changes to the bill.
However, Bush lost the support of the senator who vied with him for the 2000 Republican presidential nomination.
The Democrats want to eliminate provisions that would help vaccine producers, airport security companies and other industries.
The Senate was expected to vote as early as Tuesday to approve the bill, which would merge 22 agencies and 170,000 workers into a Department of Homeland Security. The measure was Bush's top remaining legislative goal for the lame-duck Congress, which hopes to end its postelection session this week.
First, the Senate planned a Tuesday morning showdown over a Democratic drive to delete sections of the bill that would provide legal protections to makers of airport screening equipment, to airport security firms and to other groups that Democrats consider GOP special interests. Republicans said Congress routinely offers legal shields to producers of security products during wartime.
A provision of the bill also would protect pharmaceutical companies from lawsuits over vaccines they create and their side effects.
With Democrats clinging to a 50-49 majority, counting independent James Jeffords of Vermont, every vote counted and interests ranging from the U.S. Chamber of Commerce to Public Citizen, the consumer advocacy group, was weighing in.
Complicating the White House's chances, Sen. John McCain, R-Ariz., said he would support the Democratic amendment. McCain, who unsuccessfully challenged Bush for the White House nomination, was unhappy that the pro-industry provisions were added "without proper deliberation," said spokesman Marshall Wittmann.
Bush called at least two senators Monday, including undecided Sen. Ben Nelson, D-Neb., to ask him to oppose the Democratic changes, said the senator's spokesman, David DiMartino. Nelson is a moderate who helped break a two-month stalemate over the legislation last week by saying he would support it.
Also making calls was Homeland Security Director Tom Ridge and Health and Human Services Secretary Tommy Thompson. Ridge is said to be Bush's choice to head the new Cabinet-level department once it is formed.
White House officials signaled they would paint a vote to change the bill as an effort to slow its progress. They used that tactic successfully during the fall congressional campaigns, in which the GOP captured Senate control and enlarged their House majority.
"This remains the highest priority for this lame-duck Congress," said White House spokesman Scott McClellan. "We would hope that there would not be action taken that could stop this bill from getting done."
Besides Nelson, Republicans were hoping for support from Sen. Mary Landrieu, D-La. She faces a run-off election Dec. 7 in which some Democrats worry she might be hurt if their party continues slowing work on the homeland legislation.
Her fellow Louisiana Democrat, Sen. John Breaux, will support the Democratic amendment, said spokeswoman Bette Phelan. Breaux joined Nelson last week in supporting the overall bill.
Republicans were also in danger of losing Sen. Lincoln Chafee of Rhode Island. They seemed certain to win the support of Sen. Zell Miller, D-Ga., a co-sponsor of the underlying GOP bill.
Sen. Dean Barkley, an independent from Minnesota, has not announced his position but has talked frequently with White House lobbyists, said his aide, Bill Hillsman.
"We're hopeful, but we will need some Republicans to join us" to win, said Ranit Schmelzer, spokeswoman for Senate Majority Leader Tom Daschle, D-S.D.
Hoping to accuse Daschle of further delaying the legislation, aides to House Speaker Dennis Hastert, R-Ill., and No. 3 House GOP leader Tom DeLay of Texas threatened to bring the House back into full session if the Senate should make important changes to the bill.
"Daschle's continued obstruction ignores the American people's unmistakable demand to grant President Bush the authority to strengthen the country," DeLay said in a written statement.
Senate aides said the full House will have to approve the bill again anyway, although it could be done quickly by voice vote without many members present. That is because the Senate bill corrects a pair of technical errors in the House legislation, such as one section that mistakenly refers to an aviation security law instead of the homeland security bill.
The Associated Press contributed to this report.