What happened to John Anderson here last week was a vivid demonstration of the limits of media politics. It showed the relevance of a political party -- even in a non-party age.

Anderson came here fresh from his strong showing in the Baltimore panel interview with Ronald Reagan. His manager, David Garth, ordered "crowd events" for the post-debate day, seeking television and newspaper coverage that would suggest the long-shot Independent candidacy had acquired a fresh burst of energy and support.

By holding a noon event in an outdoor plaza in the heart of Chicago's loop, always thronged with pedestrians, the Illinois congressman was able to draw a crowd of 2,000 that looked healthy to reporters and T.V. interviewers.

But the evening here was a dispiriting windup to what should have been a dynamic day for Anderson. His backers booked the 3,500-seat Civic Center and filled only about 500 of the chairs -- a failure that was highly visible on television. The Philadelphia inquirer headlined, "Empty Hall Swallows Anderson Momentum."

Embarrassed Anderson aides blamed the bust on competition from the Eagles' Monday night football game and the Phillies' appearance on television. But the basic problem was the lack of the kind of "automatic" support a political party can provide for its candidate.

The volunteer Anderson organization tried. It really tried. It bought $1,500 worth of radio spots to advertise the event -- not an inconsiderable sum for an organization whose budget depends on the daily collection of voluntary contributions. Volunteers distributed several thousand handbills promoting the event at downtown locations, and student volunteers at area campuses were pumped up to compete with each other for the number of classmates they could turn out.Key members of the volunteer network in Philadelphia and its suburban counties were asked to start a "telephone chain" that theoretically could reach thousands of Anderson fans in their homes.

In the end, the only places that turned people out were the campuses -- and only a few of them. When master of ceremonies John Buckley -- a Middlesex County, Mass., sheriff, imported to a city where no local notable is supporting Anderson -- called out the names of area campuses, there were cheers from the contingents from prestigious Penn and Temple and Bryn Mawr, but not from the more blue-collar St. Joseph's and Villanova.

The school cheers were reminiscent of an Anderson birthday party rally in Boxboro, Mass., last winter, when he was still seeking the Republican nomination. But the repetition of the device now, seven months later, seemed to measure the failure of the Anderson campaign to broaden its base or build organizational depth.

That is the main reason that strategists in both the Ronald Reagan and Jimmy Carter campaigns here now belive that serious attrition in the Anderson vote is bound to occur. His support grew after the debate and is relatively high here now, sustained so far by Anderson's skill in acquiring free media exposure. But the parity he has enjoyed with the major-party nominees in television news coverage is increasingly eroded as Carter and Reagan step up their advertising campaigns.

Without the kind of organizational activity that would tend to reinforce the marginally committed Anderson voters in their inclination to support the Independent, Anderson is likely to be whittled back to his hard-core supporters, his rivals believe.

The irony is that in a state like Pennsylvania, Anderson may suit the voters' natural inclinations better than either of his rivals. Pennsylvania likes to vote for progressive Republicans like Gov. Dick Thornburgh and Lt. Gov. William W. Scranton III.

But at a dinner here two nights after the Anderson fiasco, Thornburgh and Scranton were on hand to cheer -- not Anderson, but George Bush, Reagan's running mate. The ties of party loyalty pulled the kind of crowd that Anderson might well envy, and if the $80,000 raised for the Pennsylvania GOP was small by the affluent standards of today's Republicans, it would have looked like a small fortune to Anderson.

The same force of party loyalty is operating to help Jimmy Carter whittle the Anderson vote from the other flank. A political loner by inclination, Carter has reached for help to the Democratic mayors of this state -- including Philadelphia's Bill Green, who helped Ted Kennedy beat Carter last April in 68 of the 69 Wards.

Carter's campaign is also trying in closely to one of the more obscure aspirants on the ballot, Al Benedict, the candidate for reelection as state auditor. Benedict is not a man or renown, but he has a built-in organization of some 800 patronage employees -- an army more disciplined and reliable than the Anderson student volunteers.

On television, Anderson looks like a match for his rivals. But in the streets, as the Philadelphia fiasco showed, it is no contest.