One Obama ad from September, "Believe," attacked "cynics" who "don't believe we can actually change politics and bring an end to decades of division and deadlock." Perhaps too cleverly, Obama wove the obligatory disclaimer into a cognitively complex challenge: "This is Barack Obama. I approve this message to ask you to believe. Not just in my ability to bring about real change in Washington; I'm asking you to believe in yours."

The ad may indeed have helped rally Obama's supporters; one commented on a blog that "I can't wait for the primaries. I feel like a little kid waiting for Christmas." But the ad did little to change numbers. Obama failed to narrow the gap until Hillary Clinton made a late October debate stumble over immigrant drivers' licenses.

The most distinctive Obama ad for Super Tuesday, featuring Caroline Kennedy and vintage Camelot footage, didn't turn the tide in states where those endorsements should have carried weight, California and Massachusetts. Clinton carried both easily.

Strikingly, voters in most states never saw a negative ad. Though the contest was widely depicted as a death match, both sides made only tentative use of attack ads until the final week in Pennsylvania, as when Clinton hit Obama for ducking a debate.

The first 2008 ad that even approached game-changing status was Clinton's "3 a.m." spot, just before Texas voted. It was no masterpiece, like the 1964 "Daisy" ad it evoked, and it cost her a fair amount of mockery. But it did what it had to: It kept Hillary alive. And it shifted focus onto Obama's qualifications, a goal she'd been struggling all year to achieve.

At $15 million or more, ad spending in Pennsylvania—made necessary by the state's size and importance—dwarfed all previous campaigns in the state. For the first time, both sides used ads strategically, conducting a dialogue that became sharply negative over the contest's final weekend.

Taking advantage of his financial edge, Obama started weeks ahead of Hillary, whittling away at her double-digit lead, an easier task when you are able to dominate the message. (Clinton's campaign estimated Obama's Pennsylvania spending at $11.2 million and its own at $3.8 million.)

When Clinton was able to start advertising—good strategy dictates waiting until you can afford a continuous presence on the air—she offered "Scranton," emphasizing her Pennsylvania roots. Puzzled reporters wondered why Clinton would run a biographical ad at this late stage.

Everything changed on April 11, when Obama made his comments about bitter small-town Pennsylvanians clinging to religion and guns. Suddenly, Clinton's biographical pitch seemed to pave the way for a very helpful contrast.

Not trusting the news media to keep the "cling" story alive, Clinton launched "Pennsylvania" after the weekend, directly attacking Obama's remarks with rebuttals from average Pennsylvanians. Obama had given her a safe way to raise doubts that echoed the Reverend Wright controversy, too dangerous a topic for Clinton's ads to tackle directly.