Republican senators, who hope to use their new majority status to enact the president's agenda, yesterday began trying to clear a major obstacle: a mammoth, more than $400 billion spending package left over from last fall's partisan impasse.

The spending bill, among the largest in memory, will determine this year's funding levels for homeland security, education, drought relief for farmers, Medicare payments to doctors and the budgets of scores of federal agencies. Republicans have vowed to pass it quickly, as proof that GOP control of Congress can overcome the bickering that stalled dozens of initiatives last year.

But even as Republicans brought the measure to the Senate floor late yesterday, it was clear that the tight fiscal constraints ordered by the White House -- coupled with Democrats' determination to protect their own priorities -- pose a difficult challenge for the Senate's new leaders.

The legislation will finance government agencies, many federal programs and foreign aid through Sept. 30. These activities have been funded under a series of temporary measures since the current fiscal year began Oct. 1. They generally have frozen spending at last year's levels.

Top Democrats vowed yesterday to force a full-fledged debate over proposed GOP spending cuts for homeland defense, education and Amtrak subsidies. Chiding Republicans for cutting these programs while espousing tax cuts that would "leave no millionaire behind," Senate Minority Leader Thomas A. Daschle (D-S.D.) announced plans to try to restore $5 billion for homeland defense and $6 billion for education. Democrats also will propose hefty increases for tuition aid to low-income students and for the program that assists elementary schools with heavy concentrations of low-income students.

Meanwhile, an array of special interests, including the American Medical Association, the American Hospital Association, farm groups, southern electric utility companies and environmental organizations, weighed in to seek either more money or beneficial provisions. Mindful of the pressures, Sen. Ted Stevens (R-Alaska), incoming chairman of the Appropriations Committee, added $3.1 billion to the bill for drought assistance to farmers, and inserted language that would postpone reductions in Medicare payments to doctors, congressional sources said.

To placate governors in financially strapped states, Stevens also reportedly agreed to add $1.5 billion to help state and local governments comply with a federal requirement to improve flawed voting systems. The amount is nearly four times what the administration sought.

To provide the extra funds and still comply with White House budget limits, Stevens was proposing to cut all nonmilitary programs by about 1.6 percent, the sources said. Even the White House, which has been adamant about maintaining fiscal discipline, was reported to have given some ground. It approved an additional $825 million to help pay for last year's battles against forest fires in the West.

Majority Leader Bill Frist (R-Tenn.), a physician, supports the relief for doctors, a GOP Senate aide said. But Frist was under pressure from rural senators representing hospitals with low Medicare reimbursement rates to provide them with as much as $600 million in relief.

"We welcome the fact that they're looking at this now, and hope they would look at additional items as we go through the rest of the year," said Rick Pollack, executive vice president of the American Hospital Association. As the bill moves through Congress, it may quickly become a magnet for controversial amendments.

Sen. Pete V. Domenici (R-N.M.) plans to attach a reauthorization of the recently expired Price-Anderson Act, which provides backup insurance from the government in case of accidents at nuclear power plants.

Republicans were considering offering a measure authorizing the Environmental Protection Agency to extend the deadline for several cities -- including St. Louis, Dallas and Beaumont, Tex. -- to meet federal health standards for reducing smog, according to environmental groups and congressional aides. EPA officials and GOP congressional aides have discussed the proposal, which would override court rulings requiring those cities to meet the Clean Air Act regulations. Twenty-seven environmental groups this week urged EPA Administrator Christine Todd Whitman to disavow the congressional effort.

Three Democrats running for president -- Sens. John Edwards (N.C.), John F. Kerry (Mass.) and Joseph I. Lieberman (Conn.) -- may offer an amendment that would postpone a recent EPA decision to relax clean air rules regulating aging coal-fired power plants, refineries and manufacturing plants. Republicans and the utility industry say the changes in the "New Source Review" regulations are essential to encouraging expanded energy production, but opponents say they would allow rampant pollution.

Democratic aides expressed concern that Stevens may try to permit expanded snowmobiling in the Denali National Park and Preserve in Alaska. Environmental groups want provisions that would delay or modify a recent Bureau of Land Management ruling making it easier for western states and local governments to claim rights-of-way on thousands of miles of trails, mining roads and stream beds now off-limits in public lands.

A bipartisan group of lawmakers from Mississippi, North Dakota and Missouri also is pushing for water projects that critics say would damage the environment.

Staff writers Helen Dewar and Amy Goldstein contributed to this report.

Senate Minority Leader Thomas A. Daschle (D-S.D.) with a list of the Senate Democrats' legislative goals.