Would only one thing keep Mondale from running?

Ronald Reagan is only now getting started. The next presidential election is still a long way down the road. Yet Walter Mondale already is positioning himself for the race for the Democratic nomination three years hence. ''I'm obviously thinking about it,'' he told this reporter in an interview the other day.

Mondale says he's rethinking his stands on major issues. ''It's an evolutionary process,'' he says. ''There will not be a point where I will say 'Eureka! I've found my program for 1984.' But I will make certain that what I talk about relates to the future - what will serve the nation's future.''

Does he agree that there is a conservative trend running? ''It is overstated, '' he says. ''Now if you mean do people want a tight, frugal, economic policy, the answer is yes. But if you mean that people want to abandon the humanitarian approach of 50 years - no. People still back programs relating to education, the elderly, the environment, and on and on.''

What if Kennedy and Carter enter the race? ''If it's clear to me to run,'' says Mondale, ''I will run regardless of who gets in.''

''Mondale obviously is running,'' says one long-time political associate of the former vice-president. ''The only thing that could keep him from taking the final step and becoming an announced candidate would be for Reagan to look unbeatable in 1984. And that doesn't look too likely today - certainly not to Mondale.''

Mondale says he thinks there may be as many as a dozen candidates in 1984. But those close to the Minnesotan say he is fully aware of the likelihood that he will end up in a head-on, and possibly divisive and bitter struggle for the nomination with Kennedy.

One associate said of such a possibility: ''A good, sharp battle for the nomination could strengthen the party. But it could be otherwise.''

Certainly the Kennedy challenge to Carter in 1980 did little to help the Georgian. In fact, Carter was so wounded by Kennedy's attacks and the ensuing Democratic Party rift, and so spent by his campaign effort during the primaries, that he was in no shape to take on Reagan. Carter blames Kennedy for his loss of the presidency - and isn't likely to forgive him for that.

Mondale, too, was not amused by the Kennedy effort to oust Carter and him. An ally of Mondale's, obviously putting the best face on it, describes the relationship of Kennedy and Mondale in this way: ''It's an uneasy alliance. They have never been the closest of friends.''

The 1980 primaries did little to improve that relationship - particularly when Kennedy gave little more than token support for the Carter-Mondale ticket at the convention.

As Mondale positions himself for 1984 he has much more on his mind than the possibility of a clash with Kennedy.

First, Mondale must come up with a program that isn't an echo of the Democratic past - and yet doesn't sound too much like Reagan conservatism. How does he do it?

Mondale is a liberal, a disciple of the humanitarian approach of his mentor, Hubert Humphrey, and, of course, FDR. He has long favored a big role for the federal government in helping the poor and disadvantaged, and he is already sounding as though he will stick to that position - but then add that such federal programs must be carefully drafted to emphasize frugality and to avoid waste. Is that enough to stake out a clear-cut alternative approach? That's the question.

Can - or should - Mondale separate himself politically from Carter? He describes him in this way: ''Carter and I are very close. He allowed me to play a very significant role. I'm thankful to him.'' Some political observers say that Mondale cannot let himself be viewed as a ''Carter man,'' that this would be a distinct liability.

Yet Carter may even now be on the upgrade in the eyes of both his critics and the public. Hence, by election time, it might be that Mondale's identification with Carter, if stressed, would be a definite plus for the Minnesotan. Also, Mondale must watch lest any effort on his part to disassociate himself from Carter does make him look like an ingrate. Mondale has some careful walking to do here.

Finally, Mondale must show he truly has political appeal to the public as a whole. He has run good races in Minnesota. He was a persuasive surrogate for Carter last year, particularly in New Hampshire. Some observers say Mondale won that primary for Carter.

But nationally? Several years ago Mondale started out very early to win the presidency but didn't stir up much interest. He's a better speaker now, with much more experience and with much more to say. But the very people who find Kennedy dynamic and interesting tend to view Mondale as personable, likable, but a little bland.

Mondale has a distance to go. It's doubtless a smart move on his part to start early - so he can prove to the public, and to himself, that there is indeed a ''new'' Fritz Mondale.