Senate Democrats have been knocked off balance in their push for a health care overhaul, stalled by controversy over the plan’s cost, criticism of its limited impact, and the absence of their leader on the issue, Sen. Edward M. Kennedy .

The Senate Health, Education, Labor and Pensions Committee on Wednesday began writing legislation intended to solve the problem of the nation’s 46 million uninsured and reduce the growth of health spending. Ideally, Democrats thought, they would even have some bipartisan support for the bill.

But instead, the session began with withering criticism from Republicans who blasted both the cost estimates of the bill and the fact it had many incomplete sections.

“When are we going to have cost estimates?” asked Sen. John McCain , R-Ariz. “This is the most incredible markup I’ve ever been in my entire time at the United States Senate.” McCain called it “a joke.”

“How do you mark up a bill that you don’t know how much it costs when you’re trying to amend the bill because you’re concerned about costs?” asked Sen. Judd Gregg , R-N.H.

The last 24 hours have seen a barrage of criticism from Republicans over a preliminary cost estimate for the Democrats’ starter bill.

The cost estimate, called a “score” by the Congressional Budget Office (CBO), said portions of the measure would cost $1 trillion over 10 years, while accomplishing only a modest increase in insurance coverage. More than 30 million people would remain uninsured if the bill were enacted, CBO estimated, although the agency cautioned that it had not examined other proposals to expand coverage, including an expansion of Medicaid, the health entitlement for the poor, or a new government-run insurance plan to compete with private insurance.

Sen. Christopher Dodd, D-Conn. said all would be solved in the coming weeks. “We have to start the process, that’s my view, and that’s why we’re here today,” Dodd said. “We’ve left certain sections open because they’re controversial,” he said.

Dodd has been left in charge of the bill by Kennedy, the committee’s chairman, who is being treated for brain cancer.

Kennedy had been counted on to be the Democratic’s pivotal player in pushing forward the health care overhaul and his absence from the Capitol was never more apparent than on Tuesday, as Democrats scrambled with uneven results to rebut the GOP criticism and prepare for the committee to begin voting on amendments to the initial version of the bill.

Democrats and their allies said the Republican criticism was off-target, because the CBO score was based on a June 9 draft of the Kennedy bill not the current version. By the time it is finished, they promised, the bill would meet the goals the party and President Obama had set for themselves.

Democrats’ defense of the bill amounted to a plea for patience, noting that it is incomplete and likely to be significantly altered over the next week.

The Senate Finance Committee is also involved in the process of developing health care legislation and its chairman, Sen. Max Baucus , D-Mont., had planned to release his own draft of a bill this week. But he has also been trying to keep the total cost below $1 trillion over 10 years and court Republicans, both tasks that may take more time.