Some of the Pentagon's best friends in Congress are also its most knowledgeable critics, and their reaction to the Reagan administration's rich defense budget is a strange one:

It worries them a litte.

These veteran conservative legislators have no objection to increasing outlays for defense; they have fought for that for years.

But they think some of the increases Reagan has proposed -- enough to lift obligational authority next fiscal year to $222 billion, up about 25 percent with more to follow -- are excessive or misguided.

And they fear that over time these excesses -- plus the domestic spending cuts the military increases will force -- will destroy their new, hardwon congressional consensus in favor of defense.

Hear John Stennis, for example, former chairman, now ranking minority member of the Senate Armed Services Committee. Defense has no stauncher defender in the Senate. Yet Stennis is now flying cautionary flags about the cost of both procurement and recruitment, both weapons and men. "We don't get enough for our money," the Mississippian says of the weapons programs, and on recruitment, "We got enought men in uniform now," but not the skilled ones the services need.

"I told Mr. Weinberger" recalled Stennis, referring to Defense Secretary Caspar W. Weinberger, "that you're going to get your money this year. The years you've got to worry about are the following years, because if it's not handled well, there'll be a backlash."

Texas' John Tower, Stennis' successor as Armed Services chairman, sees the same problem ahead. "We're going to go after these cost overruns," he pledges. The proposed defense increase "puts us all on the spot more than before to show very prudent spending of all that money."

Sen. Sam Nunn (D-Ga.), who played a leading role in pushing Pentagon spending up to its current record peacetime levels, is another who fears that the Pentagon gets too little bang for the buck and warns that the pro-defense constituency could fall apart a lot quicker than it was built.

"It doesn't seem," said Nunn of the rising cost of individual weapons, "as if anybody in the Department of Defense is getting on top of it.

"It's taken five or six years of diligent effort to try to build a consensus on defense," Nunn continued, but it could be shattered in a year or 18 months if the Pentagon fails to halt runaway costs.

He said "a second land mine" which could go off under the pro-defense consensus is a seeming understatement of military spending for fiscal 1981 through 1986. Authority to spend is much higher than estimated actual spending in the budget documents. This suggests that spending will ultimately be much higher than advertised, raising the budget deficit; it may also mean that the defense industry will not be able to absorb the new billions efficiently, Nunn said.

Sen. J.J. Exon (D-Neb.), who joined Armed Services two years ago, said with open prairie bluntness that President Reagan and his advisers seem to be playing "show and tell" with the defense budget.

"I'm sure the Soviets are scared to death now that we're going to bring some battleships out of storage," scoffed Exon, referring to one of the budget proposals. "They'll target one or two of their missiles on them. This administration thinks that if they have kind of a show and tell, everything is going to work out fine.

"You know, they're talking 600-plus ships for the Navy; they're talking about all this sophisticated weaponry. Where's the manpower going to come from to do all this? We're throwing money at the problem" but not solving it.

Manpower costs are the most troublesome part of the Pentagon budget; like many pro-Pentagon legislators, Exon wonders aloud whether the country may not eventually have to return to a draft.

Rather than keep raising military pay, awarding bonuses and bringing back the GI bill in the belief that "we can hire our gladiators from the lower echelon of our economic structure," Exon recommended that the all-volunteer forece be given only one more year before be given only one more year beofre it is assessed for quality, not quantity.

If the quality is not there, continued Exon, the draft should be reinstituted. Given that possibility, why should the administration and Congress approve "massive legislation" this year to raise military pay? he asked indignantly.

"They're talking over there now in the Pentagon about a $10,000 bonus for an 18-year-old youngster if he'll serve in the infantry. To me, when people over there in that Pentagon are thinking such thoughts they aren't thinking about the survival of the United States. They're thinking about political survival -- not having the guts to say that maybe this isn't going to work," he said of the all-volunteer military.

"I'm really worried this administration is going to be throwing gobs of money at a problem so that they can hide, or stay away from, that hard core decision" on the draft.

Sen. Carl Levin (D-Mich.) of the Armed Services Committee is particularly sensitive to possible backlash against defense spending because he comes from job-hungry Detroit. He agreed with Nunn that the pro-defense constituency is proving to be fragile.

The best way to preserve that constituency, he reasoned in an interview, is for Congress to demonstrate that it is cutting fat in Pentagon budgets at the same time it is approving record high totals, thus freeing up money for muscle.

"There is growing resistance to spending so much on defense at the same time there is a lack of effort to get at waste" in the Pentagon budget, Levin said in giving his sense of the public mood.

While stating he supports raising spending for some weapons programs, Levin complained that the voters will rebel if the government keeps pursuing wasteful ones. He mentioned especially the elaborate MX missile system, a costly proposal that others also criticized.

Weinberger has not "given us a very adequate presentation" on where he intends to economize, and thus is asking for trouble in the months ahead, Levin said.

"I'm going to be spending a lot of my time on military procurement," vowed the Michigan senator.

"There is a perception that we've fallen behind. I think the public wants to be strong.

"Nonetheless, the public also wants us to eliminate some waste. There's no budget this size that doesn't have substantial waste in it. They see an effort made to eliminate waste and fat in domestic programs, but they don't see that same effort being made in the defense budget."

Sen. Gary Hart (D-Colo.) predicted Republicans will ultimately be forced to reconsider some of the defense spending increases they now support. He foresaw them coming under acute political pressure to cut the defense budget to balance off the federal revenue lost through their proposed tax cuts. b

If the government's budget deficit is larger than the administration has been estimating when tax cuts are proposed, said Hart, "that will make a very interesting dilemma for many Republicans, both an ideological dilemma and a practical political dilemma if they are forced to choose between cutting taxes and increasing defense spending. That's a terrible choice for the Republican Party to have to choose."