The United States Postal Service has long lived on the financial edge, but it has never been as close to the precipice as it is today: the agency is so low on cash that it will not be able to make a $5.5 billion payment due this month and may have to shut down entirely this winter unless Congress takes emergency action to stabilize its finances.

“Our situation is extremely serious,” the postmaster general, Patrick R. Donahoe, said in an interview. “If Congress doesn’t act, we will default.”

In recent weeks, Mr. Donahoe has been pushing a series of painful cost-cutting measures to erase the agency’s deficit, which will reach $9.2 billion this fiscal year. They include eliminating Saturday mail delivery, closing up to 3,700 postal locations and laying off 120,000 workers — nearly one-fifth of the agency’s work force — despite a no-layoffs clause in the unions’ contracts.

The post office’s problems stem from one hard reality: it is being squeezed on both revenue and costs.