WITH THE PRIMARIES over, one momentous question for Democrats, for Republicans and for the single most potent force of the whole primary season, the "undecideds," is whether John Anderson can offer himself as a real alternative to Jimmy Carter and Ronald Reagan, and not merely remain a repository for negative sentiments.

Running Anderson's campaign is David Garth, reckoned to be one of the shrewdest political consultants in the country. Garth ran the successful campaigns of both New York Gov. Hugh Carey and New York City Mayor Ed Kock, as well as scoring numerous other successes over the years. To get an idea of how Anderson's chief strategist views the uphill battle, we talked to Garth recently in his New York office.

GARTH: I'll guarantee you that when Kennedy gets beaten at that convention, as I expect he probably will, they are going to be looking for Anderson, unless I'm crazy.

Will Kennedy just stick with it? He says repeatedly that he won't go outside .

I don't think that anybody who wants to be a Democratic candidate for president next time around, let's say, and that's the final goal everybody has, would ordinarily go outside the party. It was Jack Kennedy who made the statement, sometimes party loyalty asks too much. It's a question whether Teddy will think in the same terms, at the same level, that Jack Kennedy did, or that he's got to play the rules of the game. My feeling is that Carter has not played the rules of the game with him. I would assume he will support the nominee somewhat less than enthusiastically.

But that a lot of his people will go to Anderson ?

If you analyze the Kennedy vote, it is almost half an anti-Carter vote. It certainly was in the states with polls which we could analyze. I assume a large part of his vote is an anti-Carter vote. So I assume that will go toward Anderson. The part of the vote that is a pro-Kennedy vote -- where that vote goes is a questionable thing.

What about the unions, UAW, etc? Is there a chance that some of them may swing to Anderson?

The current feeling now among the media and politicans is that labor won't. But I think labor may have some serious problems and there may be some thoughtful people there. For example, if the race is going to be Reagan versus Carter, and an Anderson, you have one man coming from the extreme right who is going to try for that labor vote with all sorts of phraseologies and symbolisms of the American flag. The man who has the right rhetoric is Carter, but he's the guy who has created the problems. So the moment you're into the inflation problem -- a guy can't buy his house, can't get a mortgage, get a car -- all the problems are really felt first by the blue collar worker, the labor member. And I'm not so sure they're going to buy that same campaign again from him. That leaves Anderson with a potential to reach that vote. Labor in recent years in their voting patterns have indicated you do not have to be their man for them to vote for you.

If the Republicans had a guy with a labor record and could talk in labor terms, I would say no. We would have a very difficult time. But they don't. So the middle ground is wide open for Anderson. The fact that he's an independent seems to be a plus with them.

I also think he's going to have an easier time, if by some chance we win this election, to govern the country. If you watch how the presidency has worked for the last quarter of a century, whenever a Democrat or a Republican wins, everything falls into line. All the options are known, the patterns are established. But if you have an independent president who has just won a popular vote, then you have a guy who is not locked into the Democratic or the Republican line. All of a sudden, for the first time, the White House and the presidency really have leverage. An that leverage is not limited by party affiliation.

Do you think the overall thrust, if he won, would be to reshape the Republican Party? What would be the effect on the two parties?

I think the parties would have the choice of reforming themselves, or they would then force the development of a third force. The effect of a man like Anderson winning the election has got to be a major shock to the system. And those guys are going to say to themselves, "Hey, fellas, there is a whole part of this country we aren't reaching and that really don't like us at all." All the polls indicate that the young wll turn out for Anderson. If he doesn't run they're not going to Carter or Reagan. They're not going to vote.

What is the election, as you see it, about?

Everyone of the polls -- and I'm not talking about political polls, I'm talking about economic polls -- shows that people are very, very insecure. It is not the malaise that [Patrick] Caddell describes. If it is a malaise at all, it's a malaise of leadership. They don't mind their lumps, which is interesting. What they mind is that they see no way out of it. They see a lot of easy promises that don't really make sense. Their level of sophistication in economics, incidentally, is amazing. You ask the people how long the recession is going to last, they say three years. If you ask them what the most important part of American foreign policy is, they will say a strong economy. They're far ahead of us. Far ahead of the press.

What this race is going to be about is "Is it going to be the same old crap? Is it going to be politics as usual, some guy mouthing the right words at the right time with the same party platforms which haven't been followed for a hundred years? Or is there a guy who can be a fresh force, someone who can lead the country?" Now once again it's a gamble. You have known nonentities versus an unknown entity.

The thing I like about Anderson is that he doesn't tell them what they want to hear. On the military, everyone's talking about MX and so on. That's not the problem. The army we paid for isn't working. That's where I think Anderson is going to be.

What Anderson has to harness is the percent of the country that calls itself independent. He has the part of the Democratic Party that is disenchanted with Carter and I think that is at least 30 to 40 percent, and then he has a shot at the smaller fraction, 20 percent, of the Republican Party membership. So the numbers could work out. He's going to be made or broken in many ways by how the press inteprets the reality of the candidacy. And they're having a hard time discounting him as the numbers come in.

What happens if the convention doesn't go to Carter?

We're working on the assumption it's Carter versus Reagan. If it's not, we're going to have to reassume.

When will he come on with a program, a platform?

Part of it should be done before the Republican convention and part before the Democratic convention. Anderson, in a way, has a tremendous advantage because he's a first-class intellect. The disadvantage is he's so fast and so quick. The kind of a backup you need to supply him with a platform on economics, a platform on foreign policy, really takes time. It takes a lot of people working, and that takes a couple of weeks to get in line.

We're not going to stage a phony convention. You know, a convention where he accepts the nomination. "Who the hell nominated him?" you know. But at the same time, we're going to have to go on television in the summer and enunciate an economic policy, foreign positions, a defense posture, energy. We would probably be remiss in our job not to take the last 12 conventions of either party and put on a split screen what they said and did.

What about the vice presidential candidate?

Anderson is in the position where he could pick. He could wait as late as after the Democratic convention, maybe, to get the best person. Ideally he would be a Democrat with governmental experience -- a senator, a governor, a mayor, a congressman. You really don't know the makeup. Carter could invade Iran backward and land in China by mistake. I put nothing beyond these guys.

You think that if Carter does make a move to the left he'll have difficulty?

He's got one problem: the credibility factor. They've been handling out a line. What do you think of Anderson on this vote and that vote? Anderson doesn't have a record of four years as president to defend. Jimmy Carter does. Kennedy is not a good analogy for how you run against Carter, because Kennedy always had to limp. The limp was Chappaquiddick. Anderson's not carrying that kind of baggage. And, by the way, neither is Ronald Reagan.

Do you think that if Anderson wasn't in this and it was Carter versus Reagan, Reagan would win?

Absolutely. Absolutely. And the problem is, in my opinion -- no one else agrees with this one either -- Carter has supplied the rationale for a Reagan win.

He opened the door?

It's more than that. The way they reacted with the need to avoid confrontation with Kennedy. No debates. Let's say something happens and Anderson drops out of the race. There's Reagan versus Carter. One of the advantages Reagan's going to have is that he can do a sanitized campaign. If they had gone on straight with Kennedy and debated him, beaten him on his own ground and fought him, they could go after you guys [the press] and say, "Hey, force Reagan to open up. You got to get to Reagan. He can't run a sanitized, plasticized campaign." Right now they can't say that.

Carter has now refused to debate with Anderson.

It's a bit ambiguous, like most things Carter does. On the one hand he describes Anderson's campaign as fantasy, on the other hand he refuses to debate with fantasy, and on yet another he says Anderson would hurt his candidacy, even though he would be debating two Republicans. If Anderson is going nowhere, why not debate him in the fall?

Reagan isn't such a bad debater.

He's damn good. The guy does have the actor training. He's very good on his feet. That's the kind of sensitivity that at this point Anderson does not have. I think intellectually Anderson could walk around all of them. But sometimes you don't win debates based on who's the smartest or who's the most intellectual. Reagan would be terrific in debates.

On the matter of inflation and unemployment, Anderson has a problem because a lot of his stuff is geared to inflation. How's that 50 cents a gallon tax going to sound with unemployment shooting up, with the rhetoric of hard times all over the place?

But if you get under $78,500 a year then you get money back. I don't think anybody has any of the answers. At least Anderson will do some provacative things that could work. If they don't work, then you change them. The real problem with Carter in the American people's minds is that he hasn't done anything.

Is there a possibility that this election could be thrown into the House?

Sure, there's a possibility. But my personal feeling is that if Anderson catches on, he's going to go all the way.

If Anderson wins, will this third force, or whatever you want to call it, become a third party, or will it re-fuse with the other two parties, or what?

That would depend on how both parties work with the president.

How about the money side of things?

The money side appears to be pretty interesting. They are averaging between $50,000 and $60,000 a day. I was surprised. I thought the donations would fall off after the first month. We're not going to have $29 million. But we'll have a base between $12 million and $15 million. You know that Anderson collected from more individuals than Carter and Kennedy together. I think the reservoir is there. We're not going to get the $1,000 donations, but we are going to get a lot of people who give $50 and $100 and things like that.

How real is the problem of press perception of Anderson?

It's a problem. The press is having a hard time though. Anderson just doesn't go away. And I have seen a recent change. The ones I've spoken to now have all said he has a chance. When he first declared their line was, "Aw, come on, Dave. What are you talking about?" Now they're saying he's really a factor. Some of them are even saying, "Well, I called it first." They also say now he can be a reality candidate, something they were not saying a month ago. But you never know. This is the craziest year I've ever seen.