Three years ago, when Sen. Gary Hart (D-Colo.) began what seemed a quixotic quest for the 1984 Democratic presidential nomination, he told reporters that there was good reason for him to ignore the odds and move so early.

"By 1988," he would say, "the woods will be full of Gary Harts."

What Hart discerned quite accurately and earlier than most was that the decade of the '80s would be a time of generational change for the Democratic Party. If he didn't stake an early claim to speak for that new generation, Hart knew, others would be ready and willing to appoint themselves its leader.

So far, Hart's strategy has worked. While he could not wrest the 1984 nomination from Walter Mondale, he ran well enough to secure a higher name recognition and a bigger corps of supporters than any other Democratic hopeful going into 1988. On the surface, Hart's announcement last week that he will not run for reelection in Colorado this year can be read as a sign of confidence on his part that the bold gamble of 1983 will finally pay off in a presidential nomination.

Why else would he give up his Senate seat? Well, there are three reasons, and each of them, I think, indicates underlying weakness rather than strength.

The first is financial. Hart is carrying a $3.4 million debt from 1984 and, despite strenuous efforts, was able to pare it by only $900,000 last year. To pay it off, raise a couple million more for the Colorado Senate race and then plunge immediately in 1987 into financing another presidential bid might have overreached his abilities -- especially since he has refused since 1983 to accept political action committee contributions.

The second reason is political. The Colorado Senate race offered little likelihood of producing an impressive victory and some risk of outright defeat. Hart won by only 19,206 votes in 1980 in his pro-Reagan state. The latest public poll, a December survey for The Denver Post, gave Hart only a 47 percent approval rating. That was 22 points below that for his colleague, Sen. Bill Armstrong (R-Colo.), who may have presidential notions of his own.

A close victory would do Hart no good for 1988, especially when potential Democratic rival Mario M. Cuomo is looking at a likely landslide in his reelection race as governor of New York. And with the poll showing Hart holding no more than 54 percent of the vote against any of three little- known Republican challengers, the risk of a loss could not be dismissed.

Third, Hart clearly thinks that he has a better chance to achieve distinction in the next two years by campaigning around the country than by working in the Senate -- which has not been particularly hospitable to his "new ideas."

Only Jesse Jackson among the 1984 Democratic contenders is likely to face Hart in the 1988 primaries and caucuses. But Hart can hear the footsteps of Cuomo and half-a-dozen other governors and members of Congress (all encouraged by the early withdrawal of Ted Kennedy) racing toward the 1988 starting line.

To hold the advantage he gained by running in 1984, Hart clearly figures he has to be the first nearly full-time campaigner for 1988. But there is real risk in this decision. In seeking to isolate himself once again from his competitors, in abandoning his electoral base in Colorado and his institutional base in the Senate, Hart may simply dramatize what an isolated political figure he is.

Hart is the most introspective and intellectual presidential contender since Jimmy Carter. Aloofness, not mental flabbiness, has always been his real vulnerability as a politician. Mondale was taking a cheap shot when he implied, during the primaries of 1984, that Hart was a "Gary-come-lately" to the fight for social justice or disarmament. Even more dubious was his challenge to Hart's "new ideas" theme with the taunting question, "Where's the beef?" Hart has applied himself to major issues of defense and economic policy as assiduously as anyone in the Democratic Party.

Yet Mondale's near-desperation tactics in 1984 worked. Why? Because almost no one came forward to defend Hart, the man who was then leading the chase for the Democratic nomination and threatening Ronald Reagan in the polls. No one of significant political stature and credibility felt the loyalty to Hart to refute those charges.

Equally significant was the fact that during that winter of 1984, when Hart was the hottest politician in America, you could just about count on your fingers the number of other elected officials who boarded his bandwagon. After nearly 10 years in the Senate, he was endorsed by three of his colleagues -- two of them as their second choices.

Even today, Hart's reputation among his colleagues is as a man of ideas, armed with some important skills in grass-roots political organizing, but essentially a loner. He has never achieved, or really worked hard to gain, acceptance by his peers in Congress as their leader or spokesman.

That is the third reason he is leaving the Senate, and, like the others, it reflects the vulnerability, not the inevitability, of his presidential bid.