Meanwhile, partly as a result of withdrawals from savings units in the Southwest, the Federal Home Loan Bank system faces a cash squeeze that is likely to require emergency Federal Reserve lending. [ Page D5. ] The Congressional unit's numbers are significant because they represent the first official Congressional statement that the estimates of the total cost of the savings cleanup by President Bush were too low. Overly Optimistic

Members of Congress have repeatedly voiced concern that the sketchy budget analysis of the Bush plan released so far by the White House seemed to use overly optimistic assumptions on the future course of interest rates and growth of deposits at banks and savings institutions and understate the actual cost.

In hearings scheduled for next week, top Administration officials, like Treasury Secretary Nicholas F. Brady and the budget director, Richard G. Darman, are likely to be called on to defend their estimates.

The Congressional unit said its report was preliminary because ''details of the President's plan have just recently become available, and in the absence of specific legislative language, C.B.O. is not certain about a number of features.''

But it said that based on its preliminary analysis, total Treasury funds needed in the first five years of the plan would be $31 billion, and $43 billion in the second five years. It added that it would cost an additional $5 billion to pay ''interest on the borrowings needed to finance the $31 billion Treasury contribution.'' Later Interest Costs