Lew said that means policymakers may have to consider new approaches. His comments came Wednesday at a conference organized by the cable TV network CNBC.
In his address to the conference, Lew stressed that the legislation provided strong safeguards for consumers and investors, and, at least on paper, ended the idea that any financial firm is too big not to be rescued by the government. "Banking will always involve some degree of risk-taking, and the goal is not to eliminate all risk in banking," he said. "But now if a financial firm fails, taxpayers will not have to bear the cost of that failure."
However, when asked after the speech about the effectiveness of the new rules, Lew said delaying implementation or weakening them could still hamper the effort to prevent another financial crisis.
If rules putting the law in place aren't sufficient by year's end to reduce the risk of big banks failing and endangering the financial system, Lew said, "We're going to have to look at other options." Lew didn't specify what the options might be.
Next week marks the third anniversary of the overhaul law, enacted in response to the crisis, which is intended to prevent another meltdown and a federal bailout of banks.
Hundreds of U.S. banks, including the country's biggest banks, received taxpayer bailouts during the financial crisis that struck in 2008 and triggered the worst economic downturn since the Great Depression of the 1930s.
Lew said Wednesday the Obama administration, like a group of senators who recently proposed legislation that would break up banks, wants to ensure that risky banks can't bring down the system. He didn't specifically endorse the legislation.
The legislation proposed by a bipartisan group of senators would force banks to split off their conventional lending and deposit-taking into separate companies from investment banking and other riskier activities.
Federal Reserve Chairman Ben Bernanke, testifying on Fed interest-rate policy to a House committee Wednesday, was asked about the regulation issue. "I think there's more work to be done before we feel completely comfortable" that the risk has been ended of big financial firms collapsing and endangering the system, he said.
Thanks to the overhaul rules that have been put in place, that possibility is less likely, but "I wouldn't be saying the truth if I said that the problem is gone. It is not gone," Bernanke said.
Bernanke and Lew pointed to new "tools" regulators have gained under the rules that reduce the possibility of big financial firms toppling and requiring federal bailouts. They include the Federal Deposit Insurance Corp.'s authority to step in to dismantle large failing firms.
Lew said that speeding up writing rules for the overhaul law has been a high priority for him since becoming Treasury secretary in February. He heads the Financial Stability Oversight Council, a group of top regulators charged with monitoring risks to the financial system.
"An awful lot of work has been done. An awful lot of work remains," Lew said. He noted that in recent weeks the bank regulators increased the capital all large banks must hold as a cushion against risk and moved toward making eight of the largest U.S. banks meet a stricter measure of financial health.
The "core elements" of the overhaul law will be largely in place by the end of the year, Lew said. He said it was especially important for regulators to finalize the so-called Volcker Rule, which would prohibit banks from trading for their own profit.
The latest version of the rule, named after former Federal Reserve Chairman Paul Volcker, includes an exemption for banks to make such trades when they are used to "hedge," or offset, other risks the bank is taking. Adoption of the rule has been delayed largely because of the banks' objections.