WASHINGTON — Senate Democrats yesterday blocked a Republican bill intended to make health insurance more affordable for small businesses, handing President Bush the second defeat in a week for his healthcare agenda.

The 55-43 vote, largely along party lines, was short of the 60 votes needed to overcome a procedural challenge and move the legislation forward. On Monday, the Senate had blocked Bush's long-sought caps on jury awards in medical malpractice cases.

The dead-end turn for the small-business bill is the impasse in Washington over healthcare, even as costs and the number of uninsured keep rising. The debate pitted companies trying to obtain broader access to coverage through lower premiums against advocates for patients trying to protect hard-won current benefits.

Most of the uninsured are workers and their families, and a disproportionate share of uninsured employees are in small companies. Despite widespread agreement on the main goal of helping small businesses pool together to save on coverage costs, the specifics of the GOP bill drew well-organized opposition.

In allowing small employer groups to buy coverage across state lines, the bill would have also let them circumvent state consumer protections. These "state mandates" spell out benefits that insurers must cover and also restrain premium hikes on older and sicker workers.

A majority of state attorneys general and many insurance commissioners objected that the bill amounted to a federal power grab. The AARP, the American Cancer Society and other nonpartisan groups warned that workers and their families would lose valuable benefits, including certain types of cancer screening and treatments for osteoporosis.

The bill's supporters said they were ambushed by a "fear-mongering campaign." Its author, Sen. Michael Enzi, R-Wyo., said savings from the legislation would come from reduced administrative costs, not bare-bones benefits. He pointed out that large employers were already exempt from many state requirements and that most of those companies offered comprehensive health benefits to their workers.

Enzi rejected complaints by Democrats that Republican leaders had refused to allow amendments to improve the bill.

But the modest changes he was willing to make were not substantial enough to attract the support of Democratic moderates.

After the vote, Enzi said he would work with Democrats to seek compromises that would allow the bill to be brought up again this year. However, both sides agreed that this week represented the best chance of passing the legislation in years.