The U.S. Congress may have scope to extend debt-limit negotiations for weeks past the mid-October date when Treasury Secretary Jacob J. Lew has said the nation will exhaust its borrowing authority, according to analysts at Credit Suisse Group AG and Jefferies LLC.

Lew urged lawmakers last week to raise the $16.7 trillion ceiling by the middle of next month, and said if they don't the Treasury would be forced to use about $50 billion in cash to fund the government. Wall Street firms are trying to pinpoint when the Treasury's money runs out: Jefferies predicts sufficient funds through the end of October, while Credit Suisse sees enough cash until as late as mid-November.

A delay past Lew's timetable threatens to disrupt the world's deepest debt market by postponing or reducing Treasury auctions, according to Goldman Sachs Group Inc. Lew's estimate reflects his concern that lawmakers will repeat delays two years ago that led to the first downgrade of the U.S.'s credit rating, analysts said.