Assistant Treasury Secretary for Economic Policy Paul Craig Roberts last week predicted that there will be a "rapid decline in interest rates" within six months provided President Reagan's program is passed and the Federal Reserve keeps a steady reign on the money supply.

In an address to the National Mortgage Banking Conference here, Roberts sounded the party line on Reaganomics. "In this policy it is not a question of consuming less absolutely in order to save more.You can do both at once if the level of disposable income grows. You can consume more and save more."

Roberts used these arguments to explain why the Reagan administration has no immediate plans to support legislation for savings incentives or for emergency aid to the ailing savings and loan industry.

"The Treasury is not an unlimited resource," he said. "It cannot cover everything that might happen." He added that the Treasury does not expect the collapse of saving and loans at this time. He also contended that the existing mechanism of government agency insurance in case of failure will be adequate.

The administration spokesman's words of optimism were sounded against a background of pessimism. Like the rest of the housing industry mortgage bankers -- who originate and sell residential loans in the secondary market -- are suffering.

Residential and income property loan originations were down 21 percent in 1980 from the previous year. During the first quarter of this year new loans amounted to only a fraction of one percent more than last year's loans. In view of the situation, the first general session of the conference was entitled "Coping in Tough Times."

Mortgage bankers shared with one another their formulas for survival through such methods as cutting personnel and diversifying products.

At a later session, five mortgage bankers presented contrasting views of the situaiton in their locale. The bleakest outlook came from Robert J. Mylod, president of Advance Mortgage Corp., a nationwide mortgage banking company with headquarters in Detroit.

The Motor City, he said, has not had a worse year since the Great Depression. Permits for housing starts this year are down to 7,000 on an annual basis, or one-third of what they were in previous years. Sales of existing housing are down 12 percent from the lows set in 1980. Starts in Chicago have plummeted from 47,000 last year to 14,000 this year, according to Mylod. Cleveland is down to 3,000.

In the mid-Atlantic states, Mark Posnick, vice chairman of Margaretten & Co. of Perth Amboy, N.J., said business was off by 33 percent to 40 percent. Half of all loan applications now have to be cosigned by a relative or friend for the buyer to qualify for a loan, he said.