THE WHITE HOUSE completed its about-face on mass transit last week. After more than two years of ignoring the subject, President Carter told a New York audience on Tuesday that the federal government should substantially increase its spending on transit systems. The next day, administration officials let it be known that the White House has changed its mind and now favors legislation that authorizes $1.7 billion in federal funds to help complete Metro.

Someday perhaps some memoir-writer will tell us what brought about this abrupt change in attitude. It could have been the 2-to-1 margin by which the House had approved that Metro bill despite last-minute efforts by White House lobbyists to kill it. Or it could have been the political pressure that was applied by New York Gov. Hugh Carey, who is seeking more than half a billion in new federal money for New York City's transit system. Or it could have been the realization -- at long last -- that mass transit is a vital component of any energy-conservation program and of any plan to revitalize the country's big cities.

Whatever brought it about, the president's new view of the value of mass transit is helpful, even though he is tying any promises of additional funding to passage of an excess profits tax on oil companies. The problems of finding the money to build, and now to operate, Metro have shown why transit systems all over the country are in trouble. And the success that Metro has already achieved in reducing automobile traffic indicates the potential in good mass transit systems to reduce the consumption of petroleum.

The trouble with the president's insistence on tying more transit funds to an excess profits tax is that the need for those funds will exist whether or not the tax is passed. State and local governments can't do the job alone, and the competition between transit systems and highways for existing revenues is going to become more, rather than less, intense. While many states have gotten over the folly of building even more new highways, almost every state is lagging in the repair of roads already in use. The bill for the proper maintenance over the next decade of both the interstate highway system and the secondary roads is going to be staggeringly high.

One proposed financing solution is the creation of a national transportation trust fund. This would be supported by the revenues that now go into the highway trust fund and the airports and airways trust fund, as well as by a portion of the proposed excess profits tax.

It would be better if Congress were to abandon the trust fund concept completely and make a new judgment on the needs of each mode of transportation with each round of appropriations. But if the trust fund idea is too well established to be eradicated, a unified fund would at least be a step toward a program in which the spending on various kinds of transportation were weighed against the nation's needs -- instead of against how much revenue a particular tax produces.