''If nominated I would run - for the Mexican border.'' That's what Arizonan Morris Udall said when some Democrats floated his name as an alternative to President Carter on the 1980 presidential ticket. Now he has kept his well-known sense of humor while deflating those who looked for a 1984 Democratic contender more liberal - or progressive, as Mr. Udall likes to say - than the front-runners so far. He says he will not seek the nomination but leave the door open to the ''highly unlikely'' possibility of a draft.
Congressman Udall cites practical reasons of time and money for his decision, along with not wishing to be a spoiler in the field. It was in tune with his brand of Southwest statesmanship to withdraw early and get out of the way of others once his decision was made. But, whatever their political leanings, Americans can only be glad that Mr. Udall does not intend to stay silent about the new clean campaign law he wants to write or the policies he wants to prevail. With all the loud pretense that campaigns tend to bring, his candid and civilized touch remains a reminder that they don't have to be that way.