US defense buildup marches on

Despite all the sound and fury on Capitol Hill, the Reagan administration's plan to ''rearm America'' is steaming ahead with all the force of a battleship. For the most part, Congress during the first three years of Mr. Reagan's first White House term has just nipped around the edges of the defense budget while approving nearly every controversial new weapons system.

Like a huge ship that has begun to turn, however, there are signs that Pentagon business practices may be changing in ways that are unlikely to be reversed. With conservative Republicans joining more traditional liberal critics , the Defense Department is being squeezed by new congressional scrutiny and restrictions.

And with mammoth federal deficits and a sharp drop in public approval for a defense buildup, Congress is likely to sit increasingly hard on the military budget this next election year.

The major unanswered questions: With so many big procurement programs already in place and developing their constituencies, will substantial cuts be possible? And what part will the continued deployment of US forces abroad play? Will the euphoria over Grenada or the doubts about Lebanon prevail?

As Congress wrapped up its work before recessing for the year, House and Senate conferees were niggling over a mere $6 billion in defense spending, less than 3 percent of the total military appropriation for 1984.

The final sum of about $250 billion is a bit less than the administration wanted, but it also represents the fourth year in a row of steady increases under Reagan (including 1981, when Congress agreed retroactively to boost defense spending).

Perhaps more important than the overall figures are the trends that have been started. Two examples: While it will be some years before the US Navy's fleet reaches the administration's goal of 600 ships, Congress has approved down payments on virtually the whole lot. Thus, Navy Secretary John F. Lehman can confidently crow that ''it's too late to stop it.''

Congress reduced the number of MX intercontinental ballistic missiles to be built during the fiscal year just begun from 28 to 21. But once started, it will be difficult to break the habit of approving the big nuclear-armed rocket. A 90 -page study on the defense budget process released this week by Common Cause explains why this is true for many weapons.

''Congressional reluctance to cancel new weapons programs requested by the Pentagon leads to a Catch-22 in the oversight process,'' writes Common Cause author Mark Rovner. ''Early in the evolution of a new weapon, Congress might be able to cancel it, but so little money is involved that it never seems worth the bother. By the time a weapon is suffering expensive cost overruns or performance problems, however, it has usually attained unstoppable political and bureaucratic momentum.''

It is no coincidence that one of the few big weapons wholeheartedly supported by antinuclear Democratic presidential candidate, Sen. Alan Cranston - the B-1 bomber - is built mostly in his home state of California. Or that liberal Pentagon critic Sen. Carl Levin (D) of Michigan reminds constituents that he has added to the defense budget $55 million for an air defense gun and armored vehicle that bring jobs to his state.

Over the next five years, the administration would like to see the annual defense budget increase steadily by two-thirds, to more than $450 billion. The request for 1985 (to be unveiled in two months) is said to be more than $320 billion. But Congress also has long-range plans for Pentagon spending, and its target figure for next year is under $300 billion. Thus, while increases in military outlays will continue, the rising curve will flatten noticeably. And the debate over the defense budget is likely to intensify, particularly as the presidential campaign heats up.

Meanwhile, Congress has moved recently to change the way the Pentagon does business. While it successfully won some escape clauses, the Defense Department now must issue warranties on new weapons. An independent office to oversee weapons testing has been established. Legislative requirements to increase the portion of weapons bought competitively seems likely. Greater protection for ''whistle blowers'' is being sought. And the Senate Armed Service Committee chairman, John Tower (R) of Texas, is looking for ways to eliminate the problems Congress itself can cause.

Most of this effort has been led by Republicans critical of Pentagon management practices. If congressional interest is maintained, such changes in the long run could have significant impact. If not, the sound and the fury could signify nothing.