WASHINGTON (AP) _ The Federal Election Commission must tighten its rules to stop congressional candidates from spending campaign money on such personal expenses as mortgages, rent, clothes and Super Bowl tickets, two watchdog groups said today.

''We believe the public record makes clear that there have been widespread abuses by members of Congress of the statutory prohibition of the conversion of campaign funds to personal use,'' said Fred Wertheimer, president of Common Cause.

He testified at a hearing of the FEC, which is considering new rules to define what constitutes personal use of campaign funds.

Wertheimer cited several alleged abuses of the law, including the spending by Sen. Ted Stevens, R-Alaska, of more than $70,000 in campaign money on cars and by then-Rep. Bob Davis, R-Mich., of $163,000 for personal expenses from 1986 to 1990.

Wertheimer said candidates should have to be able to demonstrate to the FEC that such expenses are legitimately related to a political campaign.

''The burden of proof should be placed on the candidate.'' he said.

The FEC's enforcement failure has allowed candidates to abuse existing law with impunity, said Elizabeth Hedlund, project director of FEC Watch at the Center for Responsive Politics.

''Contributors are unwittingly funding everything from Florida vacations for the candidate's family to more exotic events, such as dinners in Hong Kong, China and Australia,'' she said.

The commissioners voiced reservations about how far they can go in enforcing the law. Commissioner John Warren McGarry, for example, said that if the FEC placed the burden of proof on candidates, ''we just wouldn't get the money to enforce it.''

The FEC's budget is controlled by Congress.

The FEC's proposed rule would prohibit use of campaign money for any expense ''that would exist irrespective of the candidate's campaign or responsibilities as a federal officeholder.''

Examples of illegal uses listed in the rule include mortgage payments, utility payments, clothing purchases, tuition payments and fitness and country club memberships.

In the 20 years the law has been in place, candidates have asked the FEC its opinion on various expenditures, but the agency has not always been able to give adequate answers, FEC officials said. The six-member commission also has deadlocked 3-3 in ruling on some cases.

For example, can a candidate spend campaign money on personal living expenses?

An FEC advisory opinion issued in 1980 said campaigns may do so under certain circumstances. In the new rule, the FEC proposed limiting the use of campaign money to additional expenses incurred because of a candidacy.

To add to the confusion, some members of Congress didn't have to comply with the law until recently. Amendments made to the Federal Election Campaign Act in 1979 exempted from the law those who were members of Congress on Jan. 8, 1980.

Congress repealed that provision in 1989 and prohibited anyone serving in the current 103rd Congress from using converting campaign money to personal use.

That increased the need for a clearer law, FEC officials said.