President Clinton and congressional leaders agreed this evening to "do what is necessary" to rescue the foundering Mexican peso. A new aid package, largely crafted by Treasury Secretary Robert Rubin and Fed chairman Alan Greenspan, would come on top of an $18 billion international rescue package the Administration announced on Jan. 2. While Clinton didn't specify how much money was at stake, Administration officials privately estimated a whopping $25 to $40 billion package that, incredibly, they hoped to raise by the weekend. But instead of busting the U.S. budget, they said, the major part of the new assistance would involve loan guarantees whose costs Mexico would agree to cover. TIME Washington correspondent Suneel Ratan says the sheer amounts involved are overwhelming, but warns: "The idea of a collapsing Mexican economy is not an abstraction."