From today's WaPo poll write-up:
On taxes, an issue that often benefits Republicans and that McCain has worked aggressively to highlight, Obama holds a significant lead for the first time as voters gave the Democrat an 11-point edge on whom they trust to handle tax policy.
Nearly as many said they think McCain would raise their federal taxes as said so of Obama, an apparent repudiation of Republican efforts to portray Obama as a tax-and-spend liberal and one that follows an intense advertising barrage by Obama asserting that McCain would tax health-care benefits.
If Obama's advertising has already reversed McCain's advantage on a traditional GOP strong-suit like taxes, imagine what'll happen when he really starts spending money. After all, the expectation is that he raised in the neighborhood of $100 million last month, while McCain seems to have blown through a lot (if not most) of the $84 million he gets under the public financing system.
Update: This brings to mind a way Obama could further depress morale at McCain HQ: Announce a plan to recapitalize troubled banks using his own campaign funds. You can imagine the campaign statement: "Barack Obama today pledged $100 million of his own campaign money toward resolving the financial crisis, conditional on his opponent, John McCain, doing the same..."
Another advantage: Any time McCain accused him of playing dirty, Obama could blame it on McCain's refusal to go along with his joint recapitalization plan.