President Barack Obama’s re- election campaign is likely to have more money than any presidential campaign in history. Republican Mitt Romney’s campaign, when you factor in the super-PACs supporting him, could have even more money than that.

Both candidates will, in other words, have more than enough money to get out their message, attack their opponent and support their ground game. Even as they’re spending all this money on paid media, the campaigns will receive an almost infinite amount of free media from newspapers, television, magazines and blogs that will spend the next seven months doing nothing but covering the presidential campaign.

Yet, at the presidential level, money isn’t everything. In fact, sometimes it’s not even the main thing. Note that Rick Santorum, who was outspent many times over by Romney, nevertheless bested him in a number of Republican primaries.

Just as youth is wasted on the young, money is wasted on the rich. Money is least useful in contests where news coverage is most intense and opinions are most entrenched. How many people do you know who still aren’t sure what they think of Obama? Or are undecided about Romney? Probably not many.

But how many people do you know with a strong opinion on their congressman? Or on his or her challenger? Do you even have a strong opinion on your congressman? That’s the kind of “low- information” race where money can have a big impact.

Negative Airdrop

Although the effect of super-PACs on the presidential race will probably be limited, I worry when I read that casino mogul Sheldon Adelson plans to pump millions into a super-PAC dedicated to influencing the outcome of congressional elections. That’s where an airdrop of a million dollars in negative ads in the waning weeks of a campaign can completely change the result.

In a recent episode of the public radio program “This American Life,” Ben Calhoun tells the story of Ami Bera, a long-shot Democratic candidate who ran against Republican Representative Dan Lungren of California in 2010. Lungren has been a fixture of California politics for 30 years. Bera, a newcomer, wasn’t supposed to have a chance. He began the race 30 points behind, but he kept closing the gap. And closing the gap. Soon, he was trailing Lungren by single digits.

Then American Crossroads, the Republican super-PAC founded by Karl Rove, jumped into the campaign in the final two weeks. It dumped $680,000 into negative ads against Bera. An unknown quantity to many in the district, Bera had no time or money to respond to the attacks. He just took the hit. And lost.

Without American Crossroads, would Bera have won? Perhaps not. But it didn’t help. “It clearly had impact and moved us backwards,” he told Calhoun. “It went from a single-digit race to 14 points down. That’s a lot to make up in a single week.”

A similar barrage leveled against a presidential candidate would be far less effective. Romney knows a lot of rich people. So does Obama. If in the final days of the presidential campaign some hedge fund billionaire begins a multimillion-dollar assault on Obama, some Hollywood billionaire will probably help the president out. Either way, the ads would have a limited effect. By the end of the presidential campaign, most voters will have made up their minds. They’re not waiting for one more black-and- white clip narrated by another grim voice to push them over the edge.

In contrast, even at the end of the campaign, many potential voters will know very little about their congressional candidates. They will be susceptible to ads telling them terrible things. Some of those candidates won’t have the resources to fight back.

Pays To Listen

No one knows that better than the candidates themselves. Both incumbents and potential challengers realize that a deep- pocketed PAC could decide their race. So when they get a call from that PAC’s director urging them to support this or that, they’re that much likelier to listen. The result, then, isn’t just that moneyed interests can throw congressional elections. It’s that they wield more influence after the election — and they can exercise that power without spending a dollar.

Imagine a super-PAC funded by financial interests — “United for Economic Growth,” say — that, seeing tax reform legislation on the horizon, makes it known that it will spend $500,000 or more against candidates who support limiting the deductibility of corporate debt. That’s a small enough issue that most Americans don’t follow candidate positions on it. It’s an issue where there isn’t an organized set of interests on the other side. And it’s an issue where most politicians themselves don’t have very strong or even developed opinions. My guess is United for Economic Growth would get its way in Congress without having to spend much money at all.

Our ideas can save democracy... But we need your help! Donate Now!

Follow Ezra on Twitter @ezraklein. Ezra Klein is the founder and editor-in-chief of