NOW THAT the new Hart Senate office building is nearly complete -- in all its 16-foot-ceilinged marmoreal splendor -- no one seems to want to move into it. Many senators suspect that budget-conscious voters view the edifice as a $138 million -- and growing -- affront. Others complain that, despite the high cost, appointments are tacky and office arrangements unsuited to senatorial needs. How could 10 years of closely supervised planning and building produce such a monumental embarrassment?

The Hart building, after all, has not been without its critics -- over the last several years roughly half of all senators have cast votes strongly disapproving the building. Surely somewhere along the way someone questioned, for example, whether 16-foot ceilings would really inspire dignity among the building's occupants, as the architect claims. Or, if they would, whether 32-foot ceiling wouldn't inspire twice as much -- perhaps a good investment for the nation.

No, the Hart building is not a case of blind error. It is an example of a perhaps more disturbing political phenomenon -- the colossal mistake that everyone sees well in advance but which no one will stop until it is too late. Thus has been produced many a dam, waterway, building, bridge and, of course, weapon that was well known to be either outmoded, unneeded or unjustifiably expensive before it was even off the drawing board.

Pork-barrel politics is, of course, part of the explanation. But there is a subtler process at work. Except for a few ardent promoters, few congressmen pay continuous attention to the progress of a multi-year project. Early on in the project's life, they are assured that they are only voting for exploratory planning or analysis, and who is so narrow-minded as to block a little research? The next time they hear about it, research is in full progress, the results -- while preliminary -- are promising, but more money is needed to flesh out the plans or build an operational prototype -- or something. At the next review point, the estimated cost of completing the project will have tripled, but so much money will have already been sunk that few congressmen will be willing to admit their mistake and cut the losses.

What better example of the progress of this eyes-wide-open sort of foolishness than the Clinch River breeder reactor? Once again, last week, by a narrow vote, Congress failed to halt construction on a project that will misspend -- by most recent GAO estimates -- several billion more dollars on producing nuclear fuel that, given soaring uranium supplies and dampened energy use, won't be needed, if it ever is, until far into the next century.

Other examples abound. You can probably think of a dozen yourself. The great, gross and pretentious Hart building on Capitol Hill will be the most fitting scene imaginable in which to proceed with these white elephant projects.