Senate Democrats vowed Tuesday to fight provisions in Republican-backed legislation that they charged would reduce health insurance coverage for millions who already have it.

The bill would allow small businesses to join across state lines to buy insurance, which would give them enough clout to negotiate better rates, said Sen. Mike Enzi, the chairman of Senate's health committee and lead supporter of the measure.

"This is something that the small businesses have been asking for for almost 15 years," Enzi said.

But Sen. Edward M. Kennedy, D-Mass., said the legislation marked "a retreat from our commitment to cancer. It's a retreat for our commitment to diabetes. It's a retreat from our commitment to mental health parity." He and other critics said it would allow small businesses to purchase insurance that overrode coverage requirements mandated by the states.

Democrats said they hadn't ruled out a filibuster to block passage of the measure, which Republicans brought to the floor as part of a week-long concentration of health legislation.

Democrats blocked action on two other GOP-supported bills on Monday. Both sought to limit the amount of money that patients can receive from medical malpractive cases.

Republicans and business groups say the ranks of the uninsured would be reduced because the legislation would allow businesses to offer insurance at a lower cost. The businesses offering the policy would have to belong to same professional trade association.

But Democrats and consumer groups who oppose the bill point out that in exchange for the lower costs, insurers would be freed from state requirements that they cover mammograms, childhood physicals and diabetes, as well as other health services.

Enzi said he was open to compromises, but the bill's ultimate prospects were not clear.

The White House Office of Managment and Budget released a statement saying that President Bush supports the bill.

"By providing coverage for thousands of employees at a time, association members would be able to provide their employees better health care coverage at lower premiums," said the OMB "statement of administration policy."

Sen. Jim Talent, R-Mo., said that insurance policies for large employers already are exempt from state coverage requirements. He said it's time to do the same for small group coverage.

"The small business people are paying more to get the same benefits because they have higher administrative costs and higher overhead costs," Talent said.

But Sen. Chris Dodd, D-Conn., said he fears the insurance industry would offer plans that exclude childhood immunizations and other important services. They would also be allowed to increase premiums beyond what individuals states now allow.

"What we're doing here is shrinking the amount of benefits offered to people," Dodd said.

In recent decades, it has become common for states to require that insurance companies offer coverage for certain types of cancer screenings, such as mammograms, 49 states; colorectal examinations, 22 states; and cervical cancer screenings, 29 states.

"If you find cancer early and detect it early, you're not only going to save costs, you're going to save a life," said Daniel Smith of the American Cancer Society. "One of the barriers to going to get screened is if your insurance company won't cover it."

But insurers say the state mandates are driving up the cost of policies, and the ranks of the uninsured. States have approved more than 1,800 coverage requirements, says the Council for Affordable Health Insurance.

"Although one mandate may only increase the cost of a policy by 1 percent, 40 such mandates will price many people out of the market," the council said in a recent report.

Democrats are expected to offer a competing bill Tuesday that also would allow small businesses to band together to purchase health insurance. The Office of Personnel Management would administer the benefit. The program would be set up like the Federal Employees Health Benefit Program, said the bill's sponsors, Sens. Dick Durbin, D-Ill., and Blanche Lincoln, D-Ark.

"If it's good enough for federal employees, and it's good enough for members of Congress, there's no reason we shouldn't use this model for small businesses and self-employed people," Lincoln said.

Senate Majority Leader Bill Frist, R-Tenn., has labeled this week as "health week" in the Senate. On Monday, Democrats blocked legislation that would cap the amount of damages juries can award in medical malpractice cases. The roll calls fell well short of the 60 votes Republicans needed to advance the bill.