With a head start in fundraising and organization, Barack Obama outspent Hillary Clinton throughout the Pennsylvania primary battle. And yet, in the final week, strategic use of advertising allowed Clinton to reverse most of her losses, seize a decisive victory, and revive her campaign.

Driven by internet donations, fundraising in 2008 has dwarfed the money hunt in previous campaign cycles and pumped up the campaign industry. So it's especially strange how, until recently, campaign ads were the dog that didn't bark.

Ads—which are, in theory, the point of all that fundraising—have had little impact on most of this cycle. John McCain sewed up the Republican nomination despite having little cash for commercials. On the Democratic side, Clinton has stayed close to Obama despite his significant financial edge—at times, a 2-to-1 advantage in money raised.

A big reason for this apparent paradox is the fact that voters this year are more interested in presidential politics than they have been in a long time. Generally, what is referred to as "free media"—news stories on TV and radio and in newspapers—deliver candidates' messages to engaged, avid voters, while paid advertising is needed to reach the many others who pay little attention to political news.

This year, with more Americans actively following the campaigns, the weight of influence has been tilted toward the news media. It got to cover the intense drama of Obama's crowds, Hillary's tears, Bill Clinton meltdowns, and the many debates.

Candidates still spent heavily to broadcast ads, but they were usually bland and reflected little of their intensely combative dialogue on the campaign trail.

Clinton's early ads, for example, mostly featured jobs and other kitchen-table issues that polling identified as convincing, while Obama's ads seemed aimed mainly at his converts, often using music and fast-paced editing to convey the excitement of building a movement.

Obama's Christmas ad showcased his family; Hillary's showed her wrapping presents with labels like "Alternative Energy" and "Universal Pre-K."

Obama's ability to fire up his base often trumped Clinton's focus on using issues to woo undecided voters. His ads appeared to be secondary to his campaign's top-notch ground operation and mastery of the Web—and his own impressive ability to draw throngs.

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.

Somewhat defensively, Obama released "It Won't," using Senator Bob Casey to vouch for his harmony with small-town values. A day later, he switched to an attack of his own, "Represent," criticizing Clinton's negative campaigning without addressing his own original comments, and repeating his call for a new kind of politics just as the campaign entered its most negative phase.

From then on, the campaigns introduced new ads with dizzying frequency, breaking a basic rule of political strategy—since rapid ad traffic changes can muddy the message without ensuring that any ad truly penetrates. The Obama campaign, with its bigger war chest, may have felt it could run several ads effectively at once, releasing four ads in the final weekend and one more the day before the primary.

The candidate was having a rough time in the free media, stumbling to defend his San Francisco remarks at press events and a brutal April 9 debate; the ads offered Obama a better chance to control his message. In several, he tried to change the subject to gas prices, even as Hillary attacked him for alleged ties to energy-industry lobbyists.

By the final weekend, Obama had jumped headlong into negative territory. "Afford" harshly attacked Clinton's health-care plan (which "forces everyone to buy insurance even if they can't afford it"), while "Reason" contrasted Obama's new politics against Clinton's divisiveness and negativity.

The late attacks were risky. On the final weekend before an election, it is often too late for an ad to sufficiently reach its target, especially with a split message and fresh topic (health care). Yet Obama's upbeat image took a heavy hit for embracing negative politics.

Clinton's crisp response, "Answer," defended her health-care plan, attacked Obama's, and reminded voters of Obama's weak debate performance—all within 30 seconds, and without confusing the viewer. Another new Clinton ad, "Talk," accused Obama of taking money from drug, energy, and casino lobbyists.

Obama's reply "Exactly," hit Clinton's donations from "special interest political action committees" and "Washington lobbyists" without using voiceover and simple captions to link those entities to unpopular industries, as her ad had. "P.A.C.'s" and "lobbyists" may have only fuzzy meaning to disengaged voters; Hillary's ads show a better ear for voters' level of awareness, and sharper use of hot-button phrases.

The difference may be who's in charge of ads at each campaign. Ad guy David Axelrod directs Obama's message operation, while pollsters Geoff Garin (and until recently Mark Penn) were in charge of Clinton's strategy team. Media consultants often resist direction from polling, preferring to operate more on instinct. This might explain why Hillary's ads often seemed to target messages toward swing voters better than Obama's.

Late deciders have broken towards Clinton in many primaries. Slate's Mickey Kaus had written, "What they see on TV on Monday will be bizarrely important."

On Monday, Clinton released her most controversial ad, "Kitchen." The title referenced Harry Truman's gruff quip about the pressures of the presidency, but was also a sly dig at Obama's complaints about Hillary's "kitchen sink" attack strategy. A slicker version of the "3 a.m." ad, it challenged Obama's readiness to lead in dangerous times.

To hammer home the point, the ad included a fleeting glimpse of Osama bin Laden.

The day before an election is, of course, much too late to build up enough ratings points behind any ad and get its message across. But like the 1964 "Daisy" ad, which ran only once and was aimed at opinion leaders (much like today's Web-only ads), "Kitchen" was really a free-media play. Using Osama guaranteed the spot would make the newscasts, assisted by Obama's complaints that Hillary was resorting to the "politics of fear."

In the end, both candidates' ads showed signs of effectiveness. From a 16-point deficit prior to advertising, Obama pulled almost even with Hillary. Even after she rallied, he finished 10 points behind.

But Clinton's use of ads to drive her comeback in a much shorter space of time, while being so heavily outspent, illustrates how ad-crafting skill and message discipline can overcome a significant financial advantage—perhaps her best weapon in the contests ahead.