If you haven't heard of David McQuade Leibowitz, you haven't been in front of a television set in Bexar County, Texas. The San Antonio trial lawyer is mounting a Democratic primary challenge against Sen. Leticia Van de Putte, who won office in a special election in November. And if money is the mother's milk of politics, Leibowitz is one big, big baby: He's loaned himself $429,000 and a boatload of that money is going into television advertising.