Did Hart miss a golden opportunity to catch Mondale?

It was an opportune moment for Gary Hart. He had upset Walter Mondale in the major Ohio primary. The Mondale campaign was shaken. But did Senator Hart miss his chance for a dramatic breakthrough in the days that followed?

Some political experts, including Mr. Mondale's senior aides, say he probably did. As a result, they say, Mr. Hart won't run as well as he might have in next week's final big primaries in California and New Jersey.

Jim Johnson, Mr. Mondale's campaign chairman, says:

''I am baffled by this three-week period since Ohio. (Hart) is spending virtually no time in California. His schedule in New Jersey is not one that is well suited to winning New Jersey. And he is spending a lot of time in New Mexico and South Dakota and Colorado.''

Mr. Johnson's comments are echoed by some political insiders here in the nation's most populous state.

While Hart focused on smaller states, Mondale raced into California and got the jump on such locally important issues as the Diablo Canyon nuclear power plant.

Not only did he wrest away that issue from Hart, but Mondale also enjoyed some other, unexpected fallout from his attack on Diablo. A large crowd of union workers from the Diablo plant heckled Mondale as he spoke. The clear message that came out of the incident was that Mondale was stronger on environmental issues, and less a captive of union interests, than his critics have charged.

As the Democratic front-runners go into the homestretch for the final voting June 5, the obvious need for Hart is to change the chemistry of this race. He must convince Democrats that Mondale is a loser. He must shake the confidence of convention delegates in Mondale's ability to run strongly against Ronald Reagan. And he must convince even Mondale delegates that Hart - not Mondale - would be the far stronger candidate.

This is obviously difficult. But some political observers say it was not entirely out of reach after Hart's Ohio victory. Three things were in Hart's favor.

First, Mondale was running low on cash.

Second, two primaries were coming up that would help Hart maintain his winning streak - Oregon and Nebraska.

Finally, the biggest primary on the final day of voting was in California, a state that fits Hart's style perfectly.

Instead of pouncing on the weakened Mondale campaign and pressing his advantage in California, Hart spent most of the next week in Oregon and Nebraska - states where it was clear he was going to win anyway.

Hart did carry those states a week later by impressive margins of about 2 to 1. But in the meantime, Mondale had gotten in five solid days of campaigning in California. Mondale had also gotten the jump on Hart in New Jersey, the other big-state primary on June 5.

The results have been clear. Polls here show Mondale and Hart only a point or two apart, which in pollster parlance is ''too close to call.'' And in New Jersey, Mondale appears to be clinging to the lead.

Whatever prospects Hart had for ''blowing away'' Mondale in California seem to have passed. Yet a stunning 15- to 20-point Hart win in California might have changed the character of this entire race.

A Hart official in Washington concedes that a lot of time was poured into some smaller states. But the aide says voters in Oregon and Nebraska resent being ignored by Mondale, and that could hurt him later if he becomes the party's nominee.

The aide also notes that by June 5, Hart will have spent 13 days, or portions of days, in California, including most of the final week.