THERE IS an old joke that says if you're coming down the Pennsylvania Turnpike toward Philadelphia and you have to go to the bathroom, hold it--it will give you something to do once you get there. In sports, in education, in politics, Philadelphia has traditionally been the butt of disparaging jokes. Philly is, after all, a dead town, a city of losers. The best thing you can say for the people there is that they know when they are beat. These days, Philadelphians sag with the sickening knowledge that they are in for yet another defeat.

The losing image has been promoted mostly through the city's generally embarrassing pro teams. The 76ers set an NBA record last year for the fewest victories in a season (9), and hot shot rookies on the Eagles or Phillies invariably turn up smaller, slower and less talented than reported, and eventually fade away to play out their careers in Pottstown or Reading. There is none of the good-natured loyalty that characterized the early Met fans--Philly crowds can be ugly. They curse, they throw full beer cans, they follow their victims home. One baseball player who fell into disfavor remarked, "They would boo the loser of an Easter Egg hunt."

In this city of losers it is not surprising that Philadelphians usually have little venom left for the disappointments that are really killing them. Frank Rizzo--the supercop of 28 years who once challenged the Black Panthers to a shootout is now the mayor, owning a two-year-old mandate to clean up the streets. An enthusiastic Democrat-for-Nixon, Rizzo has been in trouble for the last two months since a secret police force was discovered investigating his enemies in Philadelphia politics. The Philadelphia Daily News asked him to submit to a lie detector test--a source of evidence he has always backed as a cop. He failed it miserably, declaring that his test was "not worth the paper it's printed on."

Last week, Rizzo initiated a campaign of attack on the press and four particularly lenient judges.

Much more frustrating has been the other law-and-order man Philadelphia embraced recently, President Nixon. Traditionally a Democratic stronghold, Philadelphia bought, part and parcel, the Nixon version of peace, economics and all-American morality. Each successive revelation of the past six months has hit the one-time supporters of both Rizzo and Nixon hard. When Nixon fired Cox and Ruckelshaus, and Elliot Richardson resigned, the last of Nixon's supporters in Philadelphia turned on him with a vengeance.

"I am surprised the kids aren't out in the streets," one former Nixon backer said. "The more I think about it, I think they should. I wish they would." And in their own way, many Philadelphians have tried to preserve their dignity with some form of protest. They write their congressmen. They sign petitions. They buy new bumper stickers saying "Now Nixon's the one" to cover the faded remains of their old one that read, "Four More Years."

BUT A FEAR of futility pervades these gestures. Pennsylvanian Hugh Scott, the senate minority leader has been overwhelmed with pro-impeachment letters, but when the White House announced that two of the tapes were missing, he simply said, "The machine age is not perfect." And the popular "honk for impeachment" demonstrations just don't have any thrill to them when the president is two full hours down I-95 or in San Clemente or Key Biscayne. No blast from a car's horn in Philadelphia can rattle the White House windows. There is no satisfaction in booing Nixon from a distance.

Without an accessible target for their outrage, Philadelphians feel lost and doubly betrayed. Whether the villain has been Dick Allen, Frank Rizzo or Richard Nixon, Philadelphians have always limited their perceptions to the individual and his personal failings. In their Watergate protest, they have thus conveniently ignored the forces which elected Rizzo and Nixon.

The political purposes for which the crimes were committed have been forgotten. No one speaks of the war crimes of Vietnam, though many of the White House horrors were associated with them. There are no news films of bombed-out Cambodian villages--only interviews with those who years ago made the decision to destroy them. The unreasonable fears, the racism and the misuses of power remain unrecognized enemies. Philadelphians are more suspicious of politicians now, but even less critical of their politics.

So there just doesn't seem to be anything to do in Philadelphia. The people there are concerned, of course, and continue to follow the Watergate hearings religiously. Searches for small-time Rizzos and Nixons have uncovered whole networks of graft, and the rascals have been mostly thrown out. But what can they do, they ask, when cops turn out to be criminals? No one is sure that the next mayor or president will not be one more disappointment.