It's probably worth taking a moment to set the record straight here.
When Obama voted against a war supplemental spending bill, it was the only option available to war opponents to change the administration's policy. Obama's position was endorsed by most Americans -- polls showed strong support at the time in favor of cutting off funding. McCain, Lieberman, and Graham, obviously, disagreed.
Except when they didn't. Last year, Senate Democrats passed another supplemental spending bill with funding for the troops that included a withdrawal timeline. McCain, Lieberman, and Graham voted against funding the troops, and urged the president to veto funding for the troops, which he did.
Does this mean McCain wanted to deliberately undermine U.S. troops during a war? That he somehow doesn't support the military? No, it means he supported troop funding when he liked the conditions of the spending bill, and opposed troop funding when he didn't. As it happens, Obama did the exact same thing, only in support of different conditions.
In July, when the RNC began airing ads on the subject, the Obama campaign issued a response:
"There are honest differences between Senator Obama's position on Iraq and Senator McCain's, but there's no question that both support our troops. Under the RNC's definition, John McCain would have also chosen politics over our military when he urged George Bush to veto funding for the troops, and we know that's not the case. This is the sort of distasteful and misleading attack from the Rove playbook that the American people are tired of, that does nothing to give our troops the equipment they need, and distracts from the honest debate we should be having about how we can keep the country secure."
What's interesting about this was that Obama could very easily turn around and say, "I can't believe John McCain voted to cut off funding for the troops in the middle of a war. How outrageous." According to the standards set by Republicans, that statement would be entirely accurate.
But Obama has decided instead to talk to voters like adults. Obama didn't cut off funding for the troops; McCain didn't cut off funding for the troops; no one actually voted to cut off funding for the troops. McCain, Lieberman, Graham, and the RNC simply have to count on public ignorance, and repeat a line they know is false.
There's an inherent risk in Obama's approach. Treating voters like grown-ups, and expecting them to see through bogus talking points, is a gamble. We'll see if it works.