Those efforts produced “a big enough loophole that a Mack truck could drive right through it,” Senator Carl Levin, the Michigan Democrat who co-wrote the legislation that led to the Volcker Rule, said Friday after the disclosure of the JPMorgan loss.

The loophole is known as portfolio hedging, a strategy that essentially allows banks to view an investment portfolio as a whole and take actions to offset the risks of the entire portfolio. That contrasts with the traditional definition of hedging, which matches an individual security or trading position with an inversely related investment — so when one goes up, the other goes down.

Portfolio hedging “is a license to do pretty much anything,” Mr. Levin said. He and Senator Jeff Merkley, an Oregon Democrat who worked on the law with Mr. Levin, sent a letter to regulators in February, making clear that hedging on that scale was not their intention.

“There is no statutory basis to support the proposed portfolio hedging language,” they wrote, “nor is there anything in the legislative history to suggest that it should be allowed.”

While the banks lobbied furiously, they were in some ways pushing on an open door. Officials at the Treasury Department and the Federal Reserve, the main overseer of the banks, as well as the Comptroller of the Currency, also wanted a loose set of restrictions, according to people who took part in the drafting of the Volcker Rule who spoke on the condition of anonymity because no regulatory agencies would officially talk about the rule on Friday.

The Fed and the Treasury’s views prevailed in the face of opposition from both the Securities and Exchange Commission and the Commodity Futures Trading Commission, which regulate markets and companies’ reporting of their financial positions. Both commissions and the Federal Deposit Insurance Corporation, which insures bank deposits, pushed for tighter restrictions, the people said.

Even some of those who have said the Volcker Rule is fatally flawed agree that, in its current form, the rule would have allowed JPMorgan Chase to do what it did.