Speaking on the Friday before the election, Sen. Barack Obama criticized a last-ditch attack from the McCain campaign, casting it as a revealing touchstone for how differently the two nominees view their political obligations.

The new McCain ad knocks Obama for his past praise of the GOP nominee. But Obama told an Iowa rally that while Beltway strategists might see his comments as making him politically vulnerable, they actually reflect a strength.

“Just this morning, the McCain campaign put out an ad that showed me praising him and Sen. Lieberman for their work on global warming — as if there’s something wrong with acknowledging when an opponent has said or done something that makes sense,” he said. “I think we need more of that in Washington!” Obama exclaimed, as if the attack spot was actually an advertisement for his bipartisan credentials.

Obama contended that Sen. John McCain is closing his campaign on a negative, personal note. “He’s spending these last weeks calling me every name in the book,” Obama said, tweaking Republicans for playing the “game” of Washington. “When you can’t win on the strength of your ideas, you make a big election about small things,” he added, reprising a line he often uses. Then Obama took a new swipe at McCain for essentially betraying his principles from past campaigns:

A couple of elections ago, there was a presidential candidate who decried this kind of politics and condemned these kinds of tactics. And I admired him for it — we all did. He said, “I will not take the low road to the highest office in this land.” Those words were spoken eight years ago by my opponent, John McCain. But the high road didn’t lead him to the White House then, so this time, he decided to take a different route. Now, I know campaigns are tough. Because we’ve got real differences about big issues and we care passionately about this country’s future. And make no mistake, we will respond swiftly and forcefully with the truth to whatever falsehoods they throw our way. The stakes are too high to do anything less.

Obama’s closing rebuttal sounds more like a condemnation than a counter-punch, reflecting confidence at this critical stage of the homestretch.