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 out of the public financing system during the primary season, after previously opting in.
It is true that both McCain and Obama have been trying to have it both ways on the public financing question. But the fault lays not so much with them as with Congress, which has failed to update the public financing system to make it a viable alternative to privately-run campaigns, and to the president and the members of the Senate, who have failed to break a deadlock over a controversial nominee to the Federal Election Commission.
You can read the remainder of my commentary on this topic here at FindLaw.