ONE NEEDED ONLY to watch how many members of Congress were sliding up to Howard Jarvis on Capitol Hill the other day to get some sense of how disinclined the lawmakers will be to pour out federal urban aid this year. So it wasn't surprising that President Carter decide against gracing this year's gathering of the U.S. Conference of Mayors in Atlanta - even though it turned out that most of the criticism there was not directed at him.

The conference (some 250 mayors with much less in common than you might think) turned into a sort of mass sensitivity session on Proposition 13 - and what to do if it rears its head in your home town. One natural reaction to voter pressures for lower local taxes is to defend your spending and then look to the state and federal governments for relief. And it is this jockeying for federal attention that has been separating the mayors into either "Sun Belt" and "Snow Belt" factions or assorted camps of rural, suburban and big-city officials.

Sen. Edward Kennedy appeared in President Carter's place. He had some advice that was hardly music to all ears. Mr Kennedy warned them, for example, that all levels of government must find "more effective ways to cut the fraud and the fat and the waste" in public programs. He urged that such cuts be made "in accord with the rising costs and a rising frustration of the taxpayers who pay the bills."

Were this coming from just another chieftain of a local taxpayers' alliance, the mayors might easily have dismissed the message. But Sen. Kennedy included the warning as part of an exhortation not to sit back and let the reaction to Proposition 13 degenderate into a broadside on the poor. He said that "none of us will find it easy to perform the sugery required," but it must be done: "If we are not willing to cut the fat, then others who care less about these urgent problems may rush in to cut the muscle, too."

That will mean finding ways to trim spending that will not seriously undermine public education or health and welfare services. The opponents of Jarvis in California should resist the temptation to retaliate by drastically clipping essential municipal services just to show supporters of Proposition 13 that it won't work. At the same time, the mayors should not brush off the message as a freak California happening. By judiciously heeding the warning and working to match federal aid with urban need, the mayors can justifiably argue for more flexibility in the use of what federal money they succeed in getting.