A month ago, Rep. Morris K. Udall (D-Ariz.) joined 72 other members of Congress in signing a letter that was a virtual declaration of war on waste.

The letter urged President Carter to stop "construction of unnecessary and environmentally destructive dams." It asked the President to "trim off the waste of tax dollars on unnecessary projects."

Five days later, the President did the unthinkable. He took the advice. He announced the end of funding for 19 major water projects - one of them, the Central Arizona Project.

Arizona voters whe regard the Central Arizona Project as the key to future water supplies in Udall's home district howled in protest.

So a "somewhat chagrined" Udall is now leading an Arizona delegation that is urging the reinstatement of the Central Arizona Project.

I have no qualification for judging whether the Central Arizona Project or any other individual project is good or bad. And it is certainly not my purpose here to single out Udall as a target for criticism. He seems to me to be one of our better legislators. Nevertheless, the incident does seem to call for comment because it is a good example of why the government always needs more money than it did last year.

Economy in government is one of the few things that incumbents and candidates agree on. Whether one is in office or out of office, he is always a volunteer in the Holy War on Waste.

It must be noted, however, that once a candidate becomes an officeholder, he adds a few words of small print to his credo. He limits his personal war against waste to the portion of the war that is being fought in the other 434 congressional districts. His own district is no longer included.

In effect, he urges everybody else to volunteer for service in the War on Waste, but if you ask him why he, too, is not in uniform, he explains: "I'm 4-F. I'd be happy to listen to your arguments against the project in my own district except that I happen to have a punctured eardrum."