The chairman of the U.S. Senate Banking Committee has warned that the new proposals

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from the administration of President Barack Obama to rein in Wall Street firms run the risk of derailing months of delicate negotiations on overhauling financial regulations.

‘‘It’s not a movable feast,’’ the chairman, Senator Christopher J. Dodd, Democrat of Connecticut, told Paul A.

Volcker, a former U.S. Federal Reserve chairman who has become an influential outside adviser to Mr. Obama.

‘‘It’s adding to the problems of trying to get a bill done,’’ he said at the end of a hearing Tuesday on the proposals, after all the other committee members had already left.

Mr. Dodd said the administration was ‘‘getting precariously close’’ to excessive ambition for the legislation. ‘‘I don’t want to be in a position where we end up doing nothing because we tried to do too much.’’ The hearing was the first formal Capitol Hill discussion of Mr. Obama’s two proposals. One would ban banks that use U.S. deposit insurance and the Federal Reserve’s discount window from engaging in proprietary trading—making market bets using their ownmoney.

The banks also would not be able to own hedge funds and private equity funds.

The other proposal would seek to bar further consolidation among financial institutions by capping the future size of any financial firm.

Both ideas have factored into a debate over how to prevent banks from becoming so large and interconnected that they are too big to fail, posing systemic risk to the financial system and requiring a government bailout if they falter.

Mr. Volcker, who led the Fed from1979 to 1987, used a vivid metaphor in calling for the establishment of a ‘‘resolution authority,’’ an idea that has attracted widespread bipartisan support.

‘‘The idea is that, with procedural safeguards, a designated agency be provided authority to intervene and take control of a major financial institution on the brink of failure,’’ he testified.

‘‘The mandate is to arrange an orderly liquidation or merger — in other words, euthanasia, not a rescue,’’ he said.

But the bulk of his testimony focused on the proposed ban on proprietary trading — what Mr. Obama has called the Volcker Rule.

Mr. Volcker tried to anticipate several criticisms of the rule. He conceded that an international consensus was needed, particularly among those nations where multinational financial institutions are based.

He said that definitions of hedge funds and private equity funds needed to be not only ‘‘carefully specified’’ to regulators but also ‘‘broad enough to encompass efforts sure to come to circumvent the intent of the law.’’ And in answer to criticism that his rule was too vague, he said, ‘‘every banker I speak with knows very well what proprietary trading means and implies.’’ For example, he said, a pattern of exceptionally large gains and losses over time in a Wall Street firm’s trading book should ‘‘raise an examiner’s eyebrows.’’ Attempts by big banks to regulate themselves will inevitably fail, Mr. Volcker said. When a bank trades for its own account—as opposed to the money of its customers — ‘‘it will almost inevitably find itself, consciously or inadvertently, acting at cross purposes’’ to the interests of its customers, he said.

The deputy Treasury secretary, Neal S. Wolin, who testified alongside Mr.

Volcker, said the proposed ban would apply to any company, foreign or domestic, that owned a bank with U.S. insured deposits but would ‘‘not disrupt the core functions and activities of a banking firm,’’ including lending, asset management, giving financial advice and hedging risks ‘‘in connection with client-driven transactions.’’ Several Republican senators expressed skepticism about the proposals.

Senator Richard C. Shelby of Alabama, the senior Republican on the banking committee, asked how regulators would discern ‘‘excessive growth’’ in a bank’s share of market liabilities.

‘‘Well, I think—the only answer I can give there is like pornography: You knowwhen you see it,’’Mr. Volcker said, paraphrasing a former Supreme Court justice, Potter Stewart.

Mr. Shelby noted that the regulatory overhaul approved by the House of Representatives in December already contained provisions permitting regulators to restrict speculative trading by banks.

But Mr. Volcker said those provisions should not allow discretion. ‘‘The regulator ends up in an impossible position during fair weather,’’ he said. ‘‘Because all the banks will say: ‘What are you talking about? Nothing is happening.

My trading is perfect. We haven’t had any big losses. You can’t restrict us. I’m going to go down to the banking committee and tell them you’re going to be unfair and unreasonable.’’’ ‘‘If you just take away the word voluntary in the House bill, I think you’ve got a better bill.’’ Senator Bob Corker, Republican of Tennessee, said that ‘‘not a single bank holding company’’ involved in proprietary trading was among the companies that collapsed in 2008 and required a U.S. government bailout. He said that fire walls already limited risk-taking by commercial banks to a degree, and he expressed concern that banks might simply move some of their activities outside the United States.

Mr. Volcker said that part of his goal was to anticipate future risks to the system.

‘‘What I want to get out of the system is taxpayer support for speculative activity, and I want to look ahead,’’ he said.