Between them, Mr. Obama and Mr. McCain are likely to contribute more to candidate flip-floppery than all of the past presidential candidates combined (although I like the term used in this article better -- "U-turns):
Mr McCain's U-turns have mostly increased his appeal to the Republican Party's base, placing him on a rightward trajectory.
Barack Obama has been performing a more traditional manoeuvre: running to the left during the primaries, when party activists need to be wooed, then shifting to the centre once the nomination is clinched.
We can expect the partisans of each side to happily point out the u-turns of the other side, but it will be nice if there are more efforts like this one that at least try to take an evenhanded approach. By the time it's over, we'll be lucky if we even know what either candidate really stands for.
Such repositioning can be tricky to make judgments about. For one thing, we should probably decide which ones might be justifiable on the grounds of new evidence (McCain on drilling, Obama on Iraq) and which ones seem to be just blatant pandering ( Obama on gun control, McCain on immigration). And what do we do about flip-flops that take a candidate from positions we didn't agree with to ones we do? Should that make us like them better, or just remind us that they're willing to abandon almost anything, even the things we care most about?