As they battled at the 11th hour for blue-collar voters in the Indiana and North Carolina primaries Tuesday, Democratic presidential candidates Barack Obama and Hillary Clinton conducted much of their debate via paid ads.

Though winning points for intellectual honesty in rejecting Clinton's proposal for a gas-tax holiday, Obama's message appeared to put him on the wrong side of the old expression that "those who can't do, teach."

As she had done in Pennsylvania, Hillary (who, unlike then, is now able to match Obama's spending) started in Indiana with an ad, "Dreams," emphasizing her regional roots. The ad was completely positive, but drew an implicit contrast to the incendiary return of Reverend Jeremiah Wright.

Obama also aired a "roots" ad, "Next Door," but was bogged down clarifying his relationship with his ex-minister, a story that dominated the news.

Clinton pivoted to her gas-tax proposal last Tuesday, showing herself as a leader who is ready to take action for the middle class. In "Trouble," about her mortgage and gas-tax proposals, she hit Obama for opposing both. It was one of the year's most effective ads.

Along with Obama, economists and newspapers slammed Clinton's gas-tax holiday as an election-year stunt that would do little to help consumers. But campaign ads are aimed at the least-attentive voters. Clarity is key—and it helps to have a simple message: I'm taking action, while my opponent says no.

Automotive issues—gas prices, insurance rates, and registration fees—are easy for voters to visualize and have galvanized several high-profile campaigns in recent years.

To denounce Clinton's plan, Obama channeled his inner law professor. Excerpting a stump speech, "Truth," he took 60 seconds to respond to Hillary's crisp 30-second message. It was heavy on abstractions about the governing process: We don't need to suspend the gas tax, we need to "change Washington."

Obama's admen clearly believe their complex message is more convincing if it's cheered on by an arena filled with sign-waving supporters. But the busy voters who see the ad while rushing to make dinner may feel less of a common bond with those rally-goers than the campaign imagines.

The Washington Post recently profiled Obama's chief strategist, David Axelrod, as a lifelong political fan now making ads for his ideal candidate. Obama's team, heavy on true believers, may have trouble inhabiting the mind-set of the politically disengaged voters they need to win over.

By Friday, Obama had sharpened his message into a much more effective 30-second spot, "Pennies." Still, before offering his own tangible solutions, such as a $1,000 tax cut, it detoured into a critique of process: Hillary's "pandering" and "poll-driven gimmickry"—not exactly hot-button issues for Hoosiers stretched by higher prices at the pump.

Likewise, Obama's citation of experts and editorials in his speeches and ads (while Hillary scorned economists who disparaged her idea in favor of their "elite" ideas) was a bit tone deaf, considering his downscale target audience. It also shifted emphasis from his own proposals.

In the 1992 Florida primary, Paul Tsongas had tried Obama's approach against a different Clinton—Bill—whom he memorably dubbed "Pander Bear" for proposing a middle-class tax cut. Despite winning nearly every newspaper endorsement in the state—featured proudly in his ads—his I'm-not-Santa-Claus campaign of hard truths lost decisively.

Democrats' happy memories of the economy during Clinton years, as Hillary reminded them on the stump, complicate Obama's efforts to portray Hillary's gas-tax proposal as one more empty promise by politicians who never deliver.

On the other hand, "changing Washington" is a pledge blue-collar voters have heard many times before, and may find dubious—particularly after the Wright controversy raised doubts about the messenger.

Later that Friday, Hillary parried with a new attack, "Gas Tax," that was again more direct than Obama's. Here, the emphasis was on making the oil companies pay for the summer tax holiday with their "windfall profits." Making the energy industry pay now is more vivid, and timely, than Obama's promise to take them on after he is elected.

On Sunday, Obama shot back with "Boost," another ad with a density problem, weighted more toward Hillary's "pandering" and "gimmicks" than his own proposals, and wedging in an explanation of the Clinton gas-tax plan's flaws.

A closing-argument spot reflected the Obama team's inability to edit. Titled "Minute," it actually took two full minutes as it vouched for Obama's ability to bring people together, recapped his gas-tax argument, and listed proposals on Iraq, taxes, and a promise to "change the world"—all set to the backdrop of another boisterous crowd.

On Monday, a day before the voting (and theoretically the best day to reach late-deciding voters), Hillary issued "What's Happened," her most direct attack. It accused Obama of opposing her gas-tax holiday because he lacks a plan of his own.

Obama responded with "Hometown," clearly prepared in advance since it made no specific rebuttal, hitting Hillary for "attacks that do nothing but harm." It was less effective than the Obama ads that were already running, and it may have been intended mainly for the final evening newscasts.

Regardless of the outcome, the ad war gave Clinton the simpler and likely stronger positioning: She offered immediate action, while Obama offered "truth-telling."

But Obama's team made their task harder by insisting on trying to educate those voters who are least interested in policy details. This explains much of Hillary's enduring advantage in the campaign's air war: She wants you to vote for her, while Obama first wants you to understand—and cheer—his logic.