President Reagan says he wants only a "modest" increase in defense spending next year of 3 percent above inflation, but the budget he sends to Congress today actually seeks an 8 percent increase above inflation, according to administration officials.

Reagan said last night he wants only a "bare minimum" increase in defense spending, which White House officials described as 3 percent. But this is based on the higher defense spending levels approved by Congress last summer, and ignores the cuts made since then, including cuts imposed by the new balanced-budget law.

When measured against the actual Pentagon spending authority this year of $278.4 billion, Reagan will be seeking an increase to $311.6 billion, officials said.

Last year, Reagan sought a 6 percent increase above inflation but settled for less than zero. This year, his senior advisers have concluded that Congress will probably not approve even 3 percent.

Nonetheless, officials said Reagan is planning a renewed sales pitch for his rearmament program, stressing the importance of his buildup for successful negotiations with the Soviets. Reagan is to give a major address to the nation on defense after the February congressional recess.

Reagan's selling effort comes against a backdrop of continued public skepticism over military spending, internal conflicts in the administration over legislative tactics and the prospect of far more severe cuts if there is a political deadlock and the balanced-budget law is triggered.

Moreover, the defense budget this year will almost certainly be determined in a larger compromise with Congress over tax increases and domestic cuts.

Reagan has rejected such a deal, but his advisers are divided over the prospects. Some White House officials, inteviewed on the condition they not be identified, said they have discussed among themselves the prospects for a "grand compromise" on defense, taxes and domestic spending, regardless of opposition from Defense Secretary Caspar W. Weinberger.

One aide compared Weinberger's approach to dealing with Congress as "holding your breath until you turn blue in the face."

But Weinberger remains rigidly opposed to any compromise, and has complained that Congress cut deeper this year than its original promise of zero growth above inflation. "We are entering a period now when our security may be held hostage to some very sharp pencils in the hands of accountants . . . ," he recently told the Detroit Economic Club.

A worrisome factor for the White House is the report of a commission on defense management that Reagan appointed last year. In late February, the panel, chaired by industrialist David Packard, is to issue its first report, on defense procurement. Further attention to the problem of expensive spare parts and Pentagon mismanagement could undermine Reagan's efforts.

But White House officials hope to use the Packard commission reports to best political advantage, offering them as evidence that Reagan will "lead the idea of reform and efficiency," said one aide.

He also said broader themes are being sought for Reagan's appeal on military spending. "There's been too much focus on the widgets," he said. "We've got to make a concerted effort to explain defense with a small 'd.' "

In past years, Reagan confronted the problem with a "threat speech," as his aides called it, saying that only a military buildup would bring the Soviets back to the bargaining table. But that approach may not work politically following Reagan's "fireside summit" with Soviet leader Mikhail Gorbachev, whom the president described as "sincere."

"It's a hell of a problem for a democracy," said one of Reagan's advisers, "to both go to Geneva, and to insist on a rearmament plan."

Six years after beginning the military buildup, lagging public support for it continues to bedevil Reagan. One reason, aides said, is that many Americans think the Soviet threat has eased after the summit, and polls show continued concern over military waste.

"There's a perception that the great menace right now is the deficit," lamented one Reagan political aide, "and the whole waste argument has been turned against the Pentagon."

For example, a Wall Street Journal/NBC News poll released last week showed that, by 50 to 44 percent, those questioned were willing to see defense spending cut to reduce the deficit. Only 38 percent felt that such reductions would hurt the nation, while 56 percent disagreed.

"My sense is that the public is on a hold-the-line approach," said Rep. Leon E. Panetta (D-Calif.). "They want to maintain strength, but not provide a blank check. They want it tightened up and they think that can be done with the spending levels you have now."

This view is widely accepted as political reality in Congress. In fact, the Congressional Budget Office has decided, in a forthcoming report, to include only the defense "baseline" showing no growth above inflation, and not a separate projection showing a 3 percent increase over inflation. This is because analysts concluded that zero real growth was the most realistic prospect for defense, officials said.