Democratic campaign changes course in Pennsylvania.

Gary Hart's aides have reached a strategic decision that could affect the remainder of the Democratic race this spring. Says a senior adviser to Senator Hart: ''We realize now that Walter Mondale is a better politician than Gary Hart.''

The adviser, of course, doesn't mean that Mr. Mondale would be a better president. But Mr. Hart's own handlers now say that the senator can't compete with Mondale in the kind of tough political slugging that took place in the New York primary.

Hart gets hurt in that kind of struggle and Mondale wins votes, the senator's advisers have decided.

Trying to brawl with Mondale in New York was just one of the mistakes that strategists say have undercut Hart's campaign and made Mondale the front-runner once again.

A source close to Hart says that from now on, the senator will try to focus on his own themes, while giving less attention to attacks by Mondale. Says this source:

''Much of the trouble began with the business about 'Where's the beef?'

''Gary was more bothered by that initially than most people realized. He felt that he had to answer it, and it took his eye off his own message.''

The ''beef'' ploy drew Hart into a losing debate that is believed to have cost him votes ever since mid-March.

There have also been some silly political mistakes that have helped put Mondale back on top. The most important of these mistakes, Hart sources say, was the Illinois television advertisement that tried to tie Mondale to the old Chicago political machine. Hart lost three ways with that ad, his own people now say.

First, Hart offended thousands of voters who are tied to the Chicago machine.

Second, Hart looked terribly ineffective when he decided to cancel the ad, but then was unable to get it stopped on a number of Chicago-area stations. It ran for at least 48 hours after he asked that it be canceled.

Third, by pulling back his criticism of the machine, he offended many of the young, middle-class professionals who normally support Hart, but vigorously oppose old-style Chicago politics.

''It was a mistake to run the ad,'' a Hart source says. ''But once he ran it, it was a mistake to apologize. Instead, Gary should have turned it around on his opponents and said: Yeah, I support the ad; it's what I believe.

''In fact, he could have gone even further. He could have said: Mondale asks, Where's the beef? Well, I'm the beef, and the Chicago machine is where the fat is.

''That kind of response would have excited his supporters, and would have also fit with his theme of old vs. new.

''Instead, all the ad did was raise questions about Gary's own ability to lead.''

When the Hart campaign rolled into New York two weeks later, the mistakes continued. Hart began battling over issues that people weren't really worked up about, like moving the US Embassy in Israel.

''He should have gone back to his central themes, gone back to Mondale as a servant of the special interests and the need to look for new solutions,'' a Hart source says.

On a trip through Northeast Pennsylvania this weekend, Hart tried to do just that. He tied Mondale to the failed policies of the past, and called for a new Democratic leadership.

''The old leadership of both parties offers no hope for our future and for our children's future,'' he told a crowd here. ''Those politicians are deaf to the lessons of the past and blind to the possibilities of the future. And they have presided over a decade of decline for Pennsylvania jobs and industry.''

This return to his themes, however, may have come too late, his own supporters say.

''It's uphill from here,'' says one insider.

In the last few days, Hart has filmed several new ads that return to his themes. Some of the ads were made by two Hart allies, Sen. Christopher J. Dodd of Connecticut and Sen. Ernest F. Hollings of South Carolina.

Old coalition politics of the kind Mondale practices, the ads say, don't solve problems. Coalition politics only results in a lot of fat in government.