The voluntary public financing system for U.S. presidential candidates, established in the post-Watergate era, is in its last throes. As it collapses, presidential candidates have been calculating--and recalculating--the advantages of opting in or out. Sen. John McCain, the presumptive Republican presidential nominee, has accused Sen. Barack Obama, the likely (though not certain) Democratic presidential nominee, of going back on his word; according to McCain, Obama had said he would participate in the public financing program in the general election. Democrats, meanwhile, have gone to federal court for permission to sue McCain for improperly trying to opt ...