This evening, one of the more influential national polls, from NBC News and the Wall Street Journal, will release its latest findings. If it reports some of the same weakness for Obama as the recent New York Times-CBS News poll did, we can conclude the race has shifted in Romney's favor.

The president’s campaign has recently pursued a campaign I heartily endorsed: to define Romney negatively. Negative campaigns are tricky business: They are a little like regimens of chemotherapy. Too little, the cancer comes back. Too much, and you can kill the person it is intended to save.

A new ad released today by the Obama campaign makes me wonder whether its strategists have made their negative cocktail in the correct dosage.

The new ad is 60 seconds long, and, in a marked shift in strategy, the president himself makes — effectively, in my opinion — the central contrast of the race: Romney represents the failed top-down policies of the past, and Obama wants to build a more secure future where the middle-class thrives.

The extra expense to buy time to air a 60-second ad — instead of the usual 30-second unit lengths — usually means a campaign has a pressing need. What is Obama’s? Is it that his negative ads have reached the end of their usefulness, and it is time to present the alternative? Or did the ads overstay their welcome and begin to backfire?

The campaign’s own polling — vastly more regular and extensive than the media’s — likely has the answer.  But the public will draw its own conclusions once the NBC-Wall Street Journal poll is out.