If President Clinton were backed into a corner and asked which member of Congress he could least afford to lose, chances are his answer would be ''Dan Rostenkowski.''
The big, gruff chairman of the House Ways and Means Committee is gatekeeper for the administration's most important legislation -- its deficit-reduction package, its health-care reforms, the Mexican free-trade pact and proposed limits on welfare benefits.
Yet it is Mr. Clinton's bad luck that Rosty is in trouble, bad trouble, as federal prosecutors close in with charges that he might have used his stamp privileges at the House Post Office to obtain illegal cash in the amount of $21,300. The sum is peanuts for a guy who could have retired last year, at age 64, with a hefty pension and an estimated $1 million in unspent campaign funds that would have been legally his.
But the chairman has stayed on to face the most important legislative challenges and the most devastating personal problems of a career that raised him from the ranks of the Daley machine in Chicago to the pinnacle of congressional power.
''My stomach is hamburger,'' he confessed last February as he was torn between the need to be a ''downfield blocker'' for a new Democratic president and the need to defend himself against the changes of ''some yahoo'' in the House Post Office.
Mr. Rostenkowski was very much the tough, controlling chairman he had always been when he steered painful Clinton tax increases through his committee without the aid of a single Republican vote and then went on to keep Democratic ranks sufficiently intact to win by six votes on the floor.
But just as he led his troops this week into conference with senators who had passed a much different tax-and-spending bill, personal scandal charges resurfaced.
Robert V. Rota, the former House postmaster, pleaded guilty to three counts of embezzlement that pointed a finger at an unnamed ''Congressman A'' -- soon identified as Mr. Rostenkowski -- as the alleged recipient of illegal cash in exchange for stamps or vouchers.
Friends and admirers were stunned and deeply worried about what the new revelations would do to the chairman personally and to the Clinton legislative program.
''Rosty is the best deficit fighter on the Hill,'' lamented the head of an advocacy group for responsible budgeting. ''I don't know what we would do without him.''
A junior member of his committee was plainly disdainful about XTC the ability of Sam Gibbons of Florida or any other ranking Democrat to step into the chairman's big shoes.
Such concerns are well-grounded. Mr. Rostenkowski was an indispensable player in passage of the landmark 1986 tax-reform law that got rid of tax loopholes which made the internal-revenue code look like a Swiss cheese. The chairman was also in the forefront of the 1990 reforms that put deficit-limiting caps on discretionary spending and decreed that fundings could not be shifted willy-nilly from domestic to defense to foreign-aid budgets.
Together, these two laws overcame some of the excesses of the Reagan era, which began with huge tax cuts that undercut the government's revenue base.
As a brand new Ways and Means chairman in 1981, Mr. Rostenkowski lost control of a legislative process that saw Democrats outbidding Democrats in a mad rush to slash tax rates.
The story of his career since has been characterized by efforts to offset that budget-busting beginning with measures to restrain the runaway growth of the national debt. This led naturally to his alliance with a president focused on reducing the size of projected deficits.
If, as it has been alleged, the administration tried to delay disclosure of the government's plea bargain with Rota that implicated Mr. Rostenkowski, its motivations are clear. The Clintonites need Rosty to uphold a House bill much more to their liking than the Senate bill. Yet if he is indicted, he will have to step down immediately under House rules.
Even Republicans dedicated to stopping the Clinton initiatives, even clean-government types appalled by the kind of tawdry dealing that went on in the House Post Office must know that if the wheels of justice bring down Dan Rostenkowski, he will not be the only loser. So will a Congress and a nation that need an effective House Ways and Means Committee to keep up the good fight against deficits.
Joseph R.L. Sterne is editor of The Baltimore Sun editorial pages.