SENATORIAL attention is always welcome when the government loses $150 billion. The current inquiry by Sen. Howard M. Metzenbaum (D-Ohio) will doubtless illuminate one untidy deal that federal regulators struck in late 1988 to put 15 failed Texas S&Ls out of business. Whether this case constitutes real scandal or merely bad administration remains to be demonstrated. But it would be a pity if voters were left under the impression that the enormous public losses were the doing of the middle-level bureaucrats and lobbyists whose names came before the senator's hearing. The Metzenbaum investigation represents, you might say, the fourth stage of the S&L disaster.

The first stage, running through 1987, was the one in which the greatest damage was done. Hundreds of S&Ls, having miscalculated the swings in interest rates, were losing enormous amounts of their customers' federally insured deposits and, in desperate attempts to save themselves, were plunging into high-risk investments that rapidly compounded the losses. Far from trying to shut down the failed institutions and stop the losses, most officials at the top level were leaning on the regulators to forbear, to bend the rules and let the bankrupts stay in the game for one more roll of the dice. The S&Ls were a prolific source of campaign contributions, and nobody wanted to say that hundreds of them were sinking out of control.

Stage Two was 1988, a time of panic-stricken recognition of the scale of the losses. But the election campaign discouraged candor, and because neither the administration nor Congress was willing to provide adequate funds, the regulators had little with which to work. To get the bankrupts out of business, they struck some peculiar deals that year.

Stage Three, which followed the election, was the public acknowledgment of the losses and a great reorganization of the regulatory system. Stage Four seems to be turning into a red-hot inquiry into the sins committed in Stage Two, the first serious attempt to curb the losses.

But those deals struck in 1988 are not the root cause of the public's loss of $150 billion, or whatever the figure ultimately turns out to be. The root cause was the bipartisan consensus, shared by the White House, the Treasury and most of Congress, against shutting down failing S&Ls promptly, beginning in 1982, as their capital vanished. Much of this larger story is on the public record. But somehow, here in Washington, the real source of the S&L losses doesn't seem to be a terribly popular subject.