PENNSYLVANIA AVENUE looked its liveliest on that day the Carter family chose to walk down it. But this revitalization of the once-grand thoroughfare was only a temporary break in the long struggle to restore permanent dignity, respectability and life to "the main street of the nation." That struggle reaches a critical point today in Congress - where a refusal to appropriate money already authorized could kill, once and for all, the public and private efforts to reverse the deterioration of the Avenue and preserve its famous hotel of Presidents, the Willard.

For those who may have lost track of the ups and downs of the Pennsylvania Avenue plan over the years since early John Kennedy days, prospects began to look up last year, when - after a thorough congressional study of financing and another round of debate - Congress authorized funds and borrowing authority for the Pennsylvania Avenue Development Corp. to use in acquiring property and making some modest improvements in the area. Such federal investment is essential to stimulate private development of necessary new offices, apartments, shops and restaurants.

In the Senate, this sensible and moderate plan - stripped of any grandiose ambitions or wasteful projects - has earned solid understanding and support. In the House, however, there has been some effort on the part of certain members to undo last year's congressional approval through a vote refusing to appropriate the money that was authorized.

The request is scheduled for consideration today by the House Appropriations Subcommittee on the Interior, headed by Rep. Sidney Yates (D-Ill.). It is not a vote on an extravagant project. As Mr. Yates and other congressional supporters of this plan have concluded after much careful debate and consideration on Capitol Hill over the years, it is a reasonable and modest commitment to the history and beauty of the nation's capital city. For a subcommiittee to destroy the plan at this point would be tragic.