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."