But the most esoteric derivatives — which also are the most profitable for banks to create and trade — have little economic purpose other than to let investors place financial bets, critics say.

A more complex type of derivative helped to inflate the housing bubble in recent years, as Wall Street repackaged high-risk mortgages into securities that speculators could use to bet on the direction of the housing market. Financial institutions earned millions of dollars in fees for creating the securities. But many of the derivatives became worthless when foreclosures skyrocketed, leading to billions of dollars of losses — and taxpayer bailouts — at the banks and insurance companies that owned them.

Image Timothy Geithner, center, Treasury secretary, with Senators Blanche Lincoln and Saxby Chambliss of the Agriculture Committee. Credit... Jim Watson/Agence France-Presse — Getty Images

Now, these obscure and largely unregulated securities — more than $600 trillion of which are tucked into investors’ portfolios, according to the Treasury Department — are at the center of the fight over financial reform led by the Obama administration.

“The best that we can do for the American people is to put in place rules that will prevent firms from taking this risk again, make sure we protect the taxpayer, bring derivatives out of the dark — that’s what we can do, ” said Timothy F. Geithner, the Treasury secretary.

The lobbying is not just coming from Wall Street. Manufacturers, airlines and other industries, which use derivatives to control their business and foreign currency costs, worry that an important means of protecting their assets could be curtailed by Mrs. Lincoln’s bill.

“I think a lot of members of Congress are just getting up to speed on how these markets work,” said Paul Cicio, who is president of the Industrial Energy Consumers of America, which represents an array of industries like fertilizers and chemicals. He said he worried that the lobbying prowess and financial resources of Wall Street firms, even when operating in the unusual environs of the agriculture committee, had the potential to outmuscle their opponents, which want greater regulation.