Will the Democrats this November be perceived as weaker than the public on national-security issues, just as the Carter administration was four years ago? Leading Democratic defense experts - those who have held senior Pentagon or White House national-security posts in the past and likely would do so in a future Democratic administration - increasingly are concerned that their party will be tagged as soft on defense. They are hoping to change that over the next few months.
''I think it's a very major problem,'' says Zbigniew Brzezinski, national-security adviser to Jimmy Carter, ''because the country by and large, while not wanting excessive spending on defense or an arms race, has a healthy appreciation of the importance of America being strong.'' There is evidence that this concern is well founded. One recent poll shows that despite all the nuclear-freeze hoopla and congressional criticism of the Pentagon budget, most Americans (70 percent) want defense spending to remain at current relatively high levels or go even higher.
''The Democrats over the course of the past decade or so have become identified as the party that's soft on defense,'' says Russell Murray II, assistant defense secretary from 1977 to 1981. ''And that, I think, is a serious weakness. I think it's important that the Democratic candidate does lay that notion aside and indicate he's for a strong defense.''
A group of military experts and former officials (some of whom are loosely organized as ''Democrats for Defense'') says the presidential campaign thus far has focused too much on what the candidates have been against, such as the MX missile, B-1 bomber, nerve gas, and ''Star Wars'' space weapons. Most of the national-security debate between Walter Mondale and Gary Hart seemed to focus narrowly on who embraced more closely the nuclear-freeze movement.
Rather, these defense experts say, Democrats should be stressing those important but nonglamorous issues like sufficient spare parts and ammunition, better airlift and sealift capabilities, as well as a moderate but steady increase in defense spending that will have to be supported for some years to come if war is to be avoided.
''It's all too easy to say - where the government's spending too much money - the one part of the federal budget that can be cut is defense,'' says Robert Komer, former undersecretary of defense for policy. ''By and large, the Democrats have tended to skirt the defense issue, aside from saying Reagan's spending too much money.''
Mr. Komer also says Democrats should point out that Defense Secretary Caspar W. Weinberger ''reigns but doesn't rule'' the Pentagon - that ''he has decentralized management of defense programs to the point where a sharpshooter - a quick and aggressive service secretary like (Navy Secretary) John Lehman - could run away with the store.''
Komer is author of a recent book, ''Maritime Strategy or Coalition Defense?, '' critical of the more offensive 600-ship fleet being formed by the Reagan administration.
Others associated with Democrats for Defense include: former Defense Secretary Harold Brown; Walter Slocombe, who worked on arms control issues at the Pentagon; William Perry, who headed defense research and engineering programs; and John Kester, a military manpower expert who has held senior defense positions.
In an article in the current issue of Foreign Affairs magazine titled ''The Politics of Vulnerability: 1980-83,'' former Navy Undersecretary R. James Woolsey says ''it has become increasingly clear to Americans that the United States is vulnerable in a sense that was never true before the advent of nuclear weapons.''
A member of the President's Commission on Strategic Forces (the Scowcroft commission), this former Carter administration defense official says that ''a bipartisan national consensus'' is needed to work through ''divisive strategic and arms control issues'' now facing the US.
In Congress, some Democrats are known as serious and thoughtful proponents of strong national defenses. At an American Stock Exchange conference here this week, Sen. Sam Nunn (D) of Georgia and Rep. Les Aspin (D) of Wisconsin argued for a steady annual military spending growth of about 5 percent (not counting inflation) instead of the ''feast or famine'' cycles more typically encountered by the Pentagon.
But so far in the Democrats' quest for the White House, says former Assistant Defense Secretary Murray, this kind of moderate advocacy has been ''sidetracked or overwhelmed by other considerations.''
Part of the Democrats' problem has to do with the nominating process. This tends to favor party activists, many of whom represent the more liberal wing of the party. In his brief campaign fling, George McGovern said he would cut Pentagon spending by a large amount. So did the Rev. Jesse Jackson.
But as shown in a recent poll by Penn & Schoen Associates (conducted for the bipartisan Committee on the Present Danger), this would not only horrify Republicans, but it would also be opposed by 63 percent of Democratic voters.
Defense has not been one of Mondale's major interests over the years. But it may be that he and his Democratic running mate will be able to accentuate the positive on defense issues rather than simply carp on the Reagan buildup - which isn't all that much larger than what Jimmy Carter had planned.
Worried Democrats, however, agree with Komer that ''so far, we haven't really come to grips with the defense issue.''