John McCain is moving ever closer to the 1191 delegate total needed to secure the nomination. (He picked up some Michigan and Louisiana delegates on Saturday, pushing him over 900.) He has only tentatively begun to attack potential foe Barack Obama, but he did find an issue on which to gain a rhetorical and perhaps tactical advantage. Friday McCain challenged Obama to make good on his promise to accept public campaign financing (Obama’s attachment to privately raised money gives you a sense of the financial upper hand the Democrats potentially will enjoy in November). If Obama declines or comes up with a preposterous counteroffer, his shiny image may get scuffed up. (The Washington Post warns him not to retreat from his prior pledge: “But this kind of backtracking and parsing isn’t what the millions of voters who have been inspired by Mr. Obama are looking for. It’s not benefitting Mr. Obama’s well-earned image as a champion of reform.”) If he accepts, an enormous financial advantage would be erased, as Obama would have to return the large sums he’s raised on his own. Either way, it seems a smart move for McCain.