The chairman of the Federal Reserve warned today that looming deficits in the Social Security and Medicare programs posed long-term threats to the nation’s economy. 

“The longer we wait, the more severe, the more draconian, the more difficult the adjustment is going to be,” the chairman, Ben S. Bernanke, said when asked at a Senate hearing when lawmakers should tackle the growth in spending in the two retirement systems. “I think the right time to start is about ten years ago.”

In substance, Mr. Bernanke’s comments were consistent with his past warnings, and those of his predecessor, Alan Greenspan, about the cost of the postwar generation retiring. But his tone appeared to be more urgent, and the timing seemed aimed at the arrival of a new Democratic-led Congress that is now setting its legislative priorities.

His comments also dovetailed with statements by Treasury Secretary Henry M. Paulson Jr. that he favored dealing with the cost of entitlements now rather than later, rejecting the advice of some Republican and Democratic lawmakers that the issues are too divisive for the new Congress to address two years before a presidential election.

Mr. Paulson has begun discussions with leading Democrats in Congress on Social Security, but the administration has not shown its hand in terms of specific proposals. President Bush’s earlier plan to convert Social Security benefits for some future retirees into individual investment accounts was rejected by Congress.

The new tone of the Congress was reflected in Senator Kent Conrad, a North Dakota Democrat who has long criticized the Bush administration’s tax cuts and warned of long-term federal deficits. Only now, Mr. Conrad was chairing the hearing and eager to hear Mr. Bernanke endorse his view that the deficits were a dangerous problem.

But Mr. Bernanke was careful not say how Congress should address the problems, despite questions from Mr. Conrad and other Democrats who oppose extending the Bush administration’s tax cuts beyond the end of the decade.

Nor did the panel’s Republican senators succeed in getting him to criticize the idea of tax increases or suggest spending cuts, as they used to be able to do with Mr. Greenspan, much to the dismay of Democrats.

Instead, Mr. Bernanke said Congress should decide appropriate spending levels for social programs, then set taxes at a level necessary to pay for the programs. He added that tax cuts may be beneficial to the economy but that they “usually” do not pay for themselves by generating more tax revenue than they drain from the treasury. 

“I’m going to try to avoid making specific recommendations on tax policy,” he said in response to a question from Senator Judd Gregg, a Republican from Georgia and the ranking minority member on the Senate Budget Committee. “I don’t think there’s a magic number. I only say that there is a difficult balance there.”

To another Republican Senator, Wayne Allard of Colorado, Mr. Bernanke referred to the cost of government programs and said: “Whatever it is, you have to pay for it. That’s what I’m saying.”

The Federal Reserve chairman came to a hearing of the Senate Budget Committee citing recent projections by the Congressional Budget Office that Social Security and Medicare outlays will rise from 8.5 percent of the economy to 10.5 percent in 2015 and to 15 percent in 2030.

These costs, in turn, would force the United States to keep borrowing, pushing the ratio of publicly held federal debt from its current level of 37 percent of the economy to roughly 100 percent in 2030, a level reached in the past only during World War II.

“If government debt and deficits were actually to grow at the pace envisioned by the C.B.O.’s scenario, the effects on the U.S. economy would be severe,” Mr. Bernanke told the senators. He said the trends would slow economic growth, drain away funds for private investment and sap confidence of consumers, businesses and investors.

Treasury Secretary Paulson’s negotiations with Congress have been very preliminary, his aides say. The talks will probably not begin in earnest until President Bush’s State of the Union speech at the end of January, when he is expected to outline not only his approach on entitlements but on the overall budget.

Mr. Bush has said, however, that the tax cuts are all but sacrosanct in his view, and Democratic leaders say they don’t expect to try to rescind them this year.

But Democrats also say that it will be impossible to deal with entitlements without looking at the tax side of the ledger, and if Mr. Bush lays out a proposal that relies solely on personal investment accounts, they will block it.

Administration officials say they are waiting for the Democrats to outline their approach.