Americans' personal income rose a modest 0.4 percent in July, the same rate as in June, prompting worries that consumers' incomes will not increase enough to produce a resurgence in economic activity this fall.

Consumer spending also rose 0.4 percent, matching the June increase, the Commerce Department reported yesterday. However, when adjusted for inflation, spending was flat in June. A comparable figure was not available for July.

"What this tells me is that consumer spending is in a holding pattern for right now," said Nariman Behravesh of Wharton Econometrics. "The second half bounce-back" in economic activity "might not develop."

The Reagan administration had expected the economy to rebound from a 1 percent growth rate in the first half to a 5 percent rate in the second half and prevent the federal budget deficit from climbing further. Private economists had predicted that economic activity would bounce back at a rate between 3 and 4 percent.

However, Behravesh said, "Everyone's going to think of revising that forecast downward. The thought was that consumer spending was going to carry us along. The likelihood is of 1 percent to 2.5 percent growth" in economic output in the second half of the year.

The Commerce Department today will release its revised estimate of gross national output in the second quarter. A preliminary report estimated the rate of growth at 1.7 percent. However, economists said they now expect that figure to be revised even lower.

The sluggish economy also puts more pressure on the Federal Reserve Board's policy-making arm, which meets today to set monetary policy for the next few weeks. The Fed had pursued an accommodative monetary policy in an effort to buoy the sagging economy, and economists don't expect the Fed to set an easier course at its meeting today.

Automobile sales have declined in recent months, while retail sales gains have been lackluster.

Disposable personal income -- income after taxes -- rose 0.4 percent in July, after falling 2.5 percent in June.

Wages and salaries, a major determinant of spending by households, had their smallest increase of the year -- $1.7 billion, compared with a $10.8 billion rise in June.

Another indicator of future sluggish consumer spending was the decline in the rate of personal saving. That rate was 3.4 percent of after-tax income in June and July, compared with a 6.1 average rate for 1984 and a 5.0 percent rate for 1983.

As savings decline, consumers become less willing to buy more goods, go further into debt or dig into their savings to finance further spending, economists said. The low 4.3 percent rate suggests that consumers will hold off spending until they can increase their savings, economists said.

Payrolls for commodity-producing industries rose $400 million, compared with $1.1 billion in June. Manufacturing payrolls rose $500 million last month, following a $1 billion increase in June.

Payrolls for service industries increased $100 million in July, compared with $5 billion in June. However, government wages and salaries increased $1.9 billion compared with $1.7 billion in June.

Commerce said that retroactive Social Security benefit payments increased last month at an annual rate of $5 billion, contributing to the large increase in income from government transfer payments. These payments resulted from the recalculation of benefits for some retired persons, Commerce said.

Commerce also reported that the distortions to after-tax income caused by the delay in processing federal income tax returns this spring finally have been eliminated.

Personal outlays rose $11.7 billion in July, compared with $11.9 billion in June. Purchases of durable goods rose $2.5 billion, following a decline of $6.4 billion in June that was attributed to a drop in new-car purchases. Purchases of nondurable goods rose $900 million in July and June. However, purchases of services rose $6.9 billion, following a $16 billion increase in July.