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.