The White House, as part of its all-bets-are-off reassessment of the fiscal 1984 budget, is challenging a 7.6 percent pay raise the Pentagon had penciled in for the 2 million men and women in uniform effective next Oct. 1, defense officials said yesterday.

The Pentagon had estimated 7.6 percent was necessary to keep the all-volunteer armed forces from dropping behind next year's likely inflation rate and private-sector pay.

In a new effort to find defense cuts, however, the Office of Management and Budget has asked Defense Secretary Caspar W. Weinberger his views on a number of pay alternatives, ranging from an outright freeze to a raise this year of only 4 percent, the same as last year.

But defense officials said yesterday there is now almost no hope for a 7.6 percent raise in the budget the president is scheduled to send to Congress Jan. 31. One reason is that the likely inflation rate now is lower than when the budget was first drawn up. Another is the soaring budget deficit, and an insistence in Congress that there be cuts in defense as well as domestic programs.

Also unclear is how large a raise Reagan will propose for the government's civilian employes next Oct. 1. They, too, got 4 percent last Oct. 1.

Pentagon manpower officials said raises of 5 percent would probably keep military people even next fiscal year with civilians holding comparable jobs. Weinberger is expected to fight any White House efforts to go much below a 5 percent raise on grounds that it would hurt the all-volunteer military force Reagan has bragged about recently.

Several developments have strengthened those OMB officials who want to cut military pay to bring down projected federal deficits of $200 billion next year and $295 billion in 1988.

For one thing, Reagan's rearmament program is at a point where canceling weapons or stretching out their production will not mean much of a spending cut next year because the payments have already been budgeted over several years under a kind of installment plan. A cut in military pay, by contrast, would show up immediately.

Every percentage point in military pay equals about $380 million. Thus outright cancellation of the planned 7.6 percent raise would save $2.9 billion. Reducing it to 5 percent would save almost $988 million and going down to 4 percent would save $1.37 billion in spending.

Another factor on the side of would-be White House budget cutters is that the Army, Navy, Air Force and Marine Corps all have more volunteers than they can accept today. The lack of jobs in the civilian economy has helped drive young men and women to the recruiting stations.

A young man or woman who enlists in the service today gets $573 a month in base pay plus room, board and other benefits. A sergeant major with 26 years in the service gets $2,215 a month; a beginning lieutenant, $1,098; a full colonel, $4,002; a full general, $5,316.

Congress, in setting budget targets for fiscal 1983, 1984 and 1985, recommended only 4 percent military pay raises each year. Reagan went along with that 4 percent rather than his recommended 8 for the fiscal 1983 raise that was effective last year, but said he did not feel bound to the same 4 percent for fiscal 1984 and 1985.

The Labor Department's Bureau of Labor Statistics surveys the private job market each year and estimates how large a raise it would take to make military pay comparable with civilian. Defense officials predict that the agency now is likely to estimate about 5 percent for fiscal 1984.

Weinberger has on his side of the argument the constant concern of the Joint Chiefs of Staff that cuts in military compensation could bring back the old days of low-quality volunteers and an exodus of the skilled technicians needed to operate, maintain and repair today's highly complicated weapons.

"The services are just starting to get well," said one Pentagon manpower specialist in warning that dropping next year's pay raise or slashing it much below 5 percent would be asking for trouble.

The White House-Pentagon review of military pay is part of the broader reassessment ordered by the president on how much is enough for defense. Deep cuts in budget authority have been studied by the Pentagon as well in the last-minute review ordered by Weinberger under White House pressure.

In what Pentagon executives described as a more flexible, if forced, position on cuts by Weinberger, the defense secretary is responding to a series of queries from the OMB on the impact of a whole range of reductions.

Vincent Puritano, right-hand man of Frank C. Carlucci when Carlucci was deputy secretary of defense, is overseeing the budget analyses and is considered a leading candidate to succeed Jack R. Borsting as Pentagon comptroller. Borsting left that post at Christmastime