After months of demurring, the Bush administration last night formally asked Congress to authorize thrift cleanup spending that would bring the tab for the coming year to $117 billion.

In a letter sent to congressional banking committees, Treasury Secretary Nicholas F. Brady said that the Resolution Trust Corp., the independent agency overseeing the cleanup, estimates it will need $40 billion to cover losses from thrift failures in 1991.

RTC officials have already estimated they will spend $60 billion next year in borrowed funds they hope to recover eventually through the sale of real estate, securities and other thrift assets. In addition, Brady asked that Congress either lift the RTC's borrowing limits or appropriate an extra $17 billion next year.

While the amount sought is roughly in line with what RTC Chairman L. William Seidman has recently said he will need, it represents the Bush administration's acknowledgment that the cleanup will cost much more than it estimated early on. The RTC was given $50 billion in the 1990 fiscal year.

Brady said in the letter that the administration would prefer a "permanent and definite appropriation" but said "we understand" if Congress rejects the idea of what some have termed "a blank check."

Said Brady: "As the end of the session draws near, the public interest requires that we proceed in a bipartisan manner to assure that the RTC continue its important work."

House Banking Committee Chairman Henry A. Gonzalez (D-Tex.) has been demanding a formal request of the administration in a flurry of letters over the past few months. The latest, also signed by Senate Banking Committee Chairman Donald W. Riegle Jr. (D-Mich.), said Congress would not fund the thrift rescue in 1991 unless it got a specific request before adjournment Oct. 19.

Some of those involved in the cleanup issue on the Hill believe the administration balked at making a specific request because it did not want to lay claim to the political costs. Several said they had expected the administration to try to slip thrift cleanup funding into whatever budget agreement emerges.

In an addendum to the now-discarded budget summit agreement, the Office of Management and Budget projects that the cleanup's heaviest drain on the Treasury will last a year longer than expected -- until 1994 -- and Treasury officials said the gloomier projections are due to the slow sale of thrift assets in the deteriorating economy.

Treasury officials said, however, that they are sticking with their most recent estimates that the long-term losses will be about $130 billion.