Daunted by what he saw as a hopeless effort to rally Congress behind his $40 billion peso rescue plan, President Clinton today abruptly invoked executive authority to pull $20 billion from a special federal fund in order toaid Mexico's tottering economy. The president then raised an additional $15 billion from international lenders. Mexican financial markets and the peso rallied after Clinton announced the moves. The bold gamble, which Treasury Secretary Robert Rubin says will avert a financial catastrophe, drew on a presidential fund that normally would be used to defend the value of the U.S. dollar. House Speaker Newt Gingrich and Senate Majority Leader Bob Dole joined top Democrats in endorsing the move. "This is an important undertaking and we believe that the risks of inaction vastly exceed any risks associated with this action," the leaders said in a joint statement. But many members of Congress were bitter: Clinton "bypassed Congress and the American people," said Rep. Steve Stockman, (R-Tx.). "The American taxpayer is still on the hook if Mexico defaults."