Commerce Department Chief Economist Robert Ortner noted that when the economy started turning sour last winter, consumers stepped in and picked up their spending to save the country from a mild recession.

"They're there when you need them," Ortner quipped. "They're always ready to bail out the economy."

Maybe not this time. Consumer spending, which rose at a 5.2 percent pace in the first half of the year, is expected to slow to about half that rate in the second half, many private economists are saying. Automobile sales have been weak since May, retail sales for the past two months have declined, and even credit card and consumer loan spending is off.

The increase in personal disposable income, the portion of incomes available for buying, has slowed, and the American job machine isn't churning out enough jobs for incomes to rise faster. Americans have dug into their savings about as far as they will go and have decided to rebuild their savings and pay off old debt.

Consumer spending comprises about two-thirds of national output, or gross national product, and thus is an important part of the nation's economic health. Automobile sales alone are 3 percent of GNP.

Although few economists are predicting a recession any time soon, they are saying that the current sluggishness in the economy will prevent the administration from reaching its goal of 5 percent growth in the second half, a rate needed to prevent the federal budget deficits from rising further and to improve the jobs outlook.

So economists are doubtful that consumers will step in to keep the economy revving this time.

"Consumer buying sentiment has weakened somewhat," Allen Sinai, chief economist for Shearson Lehman Brothers, wrote recently. "Debt loads relative to income are high and rising, and loan delinquency rates have been higher. And travel and related activities have been keeping consumers away from the stores."

"The signals from consumer spending have become less encouraging in recent months, and little improvement is likely in the immediate future," according to Data Resources Inc., a major econometric forecasting firm.

Retail sales dropped 1.4 percent in June and rose only 0.4 percent in July, providing evidence that consumer spending has slowed. That small increase "indicates only a sluggish pace of consumer spending during the third quarter," Sinai said. "The small uptick in consumption that is suggested as a result cannot provide sufficient lift to the economy for the 4.5-percent-plus growth expected by the Federal Reserve and the administration in the third quarter and the second half."

Additionally, a large part of consumers' increased purchases were imports, which didn't help the domestic economy much.

Economists point to these recent events to confirm that consumers have had enough for now:

*Consumer installment credit rose in June by the smallest amount since last November, the Federal Reserve reported last week.

*Automobile sales have slowed significantly since May when the Japanese government relaxed restraints on exports of cars to the United States and U.S. auto makers ended inventive sales programs. The reduction in interest rates, engineered by the Federal Reserve Board, didn't help much.

General Motors announced a new sales incentive program last week, including a 7.7 percent financing rate, the lowest the company has offered through its financing subsidiary, to boost sales.

*The unemployment rate has stayed at 7.3 percent for six months, as job gains have been unable to surpass the growth in the labor force. More than 220,000 factory jobs -- many of them high-paying -- have been lost since January.

*Income growth has slowed, and consumers have dug deep into their savings to keep up spending, dropping the savings rate to 3.9 percent in June. A normal rate is about 5 percent. It is unlikely that consumers will reduce their savings further to channel that money into spending.

Economists predicted earlier this year that consumers would slow their rate of spending in the second half, but they said that economic growth would rebound from the 1 percent rate in the first half because businesses would want to build up their inventories, which had been depleted during the winter.

However, recent economic statistics appear to explode that scenario, economists said. Instead of building up inventories worked off during the winter and spring, companies are finding that unwanted stocks are piling up because demand has slowed. Few companies will want to keep large stocks on hand when no one is buying them.

The one area of strength in consumer buying probably will be housing sales, economists said.

"Although mortgage rates have tended to stabilize in recent weeks, they are still well below their 1985 highs," William V. Sullivan wrote for Dean Witter Reynolds Inc. "The reduced cost of borrowing mortgage credit should provide support for the residential construction markets throughout the second half of 1985."

But, the latest statistics raise doubts about that forecast: Mortgage rates are beginning to rise, and housing starts plunged 2.4 percent last month.

The key is figuring out why consumers buy and why they stop buying. Economists are not so much concerned with the toothpaste, potatoes or work shoes that people buy as with the items that tend to be more volatile and send the economy up or down such as automobiles, major appliances and furniture.

One debate in economists' circles is over the role of near-record-high debt burdens in restricting consumer purchases.

"Fears have mounted that consumers have exhausted their ability to keep the expansion alive," according to Charles Lieberman, a vice president for Shearson Lehman Brothers and an expert in consumer behavior.

"The primary concern stems from the surge in installment debt, which suggests to many that households have become overly leveraged and are less able to sustain increases in spending," Lieberman said. "It is also alleged that the diversion of domestic demand to imports and the extraordinarily weak performance of the manufacturing sector could spread through the rest of the economy, slowing employment, income and, therefore, consumption growth.

"But is the consumer really spent as some fear?" Lieberman thinks not.

Economists often look at the relation of total installment debt to personal disposable income to predict at what point consumers will decide they have spent enough and should start paying off bills.

However, Lieberman contends that that ratio is unusually large now -- a record 18 percent -- because of unusual factors. One factor is that credit cards increasingly are used for routine transactions that do not lead to loans, Lieberman said. About 28 percent of card holders with one of the country's largest banks promptly pay off their charge balances to avoid any finance charges, but their end-of-the-month charge balances are included in the revolving credit figures.

Another 43 percent of card holders pay off a sizable portion of their outstanding debts every month, and less than 25 percent carry large balances each month.

"This distortion is compounded by the evolution of the financial structure," Lieberman said. "Relatively few individuals possessed credit cards 30 years ago, whereas most consumers have some today, and many people have so many that their wallets wear out prematurely."

Some economists dispute Lieberman and say that the high rate of debt is keeping many consumers out of stores. Another major factor is the growth of income from higher employment and higher wages on the job.

"Income growth will be the major determinant of spending behavior in the next two years" along with a mild ups and downs in interest rates, DRI said. "With no shocks on the horizon, consumer attitudes should stage no more than a controlled retreat."

During the first year of the recovery, consumers concentrated their spending on big-ticket items, including cars and furniture, and home electronics such as wide-screen television sets, video cassette recorders and telephone answering machines, DRI said.

During the second year, people bought jewelry and watches, sporting goods, toys and sport supplies, and boats and airplanes. Sales of home electronics also increased "as the rising dollar encouraged the entry of inexpensive imports," DRI said.

This year, lower interest rates should help housing, furnishings and appliances. If interest rates rise this fall, spending on those goods could slow.

In the next 10 quarters, consumers will buy more services, DRI said. "Higher interest rates and small employment gains will limit both the willingness and the ability of consumers to make major durable purchases," DRI said. Sales of radios, televisions, musical instrument and sporting goods will increase, DRI said. But most of those goods are imports.