We decided to track which candidates are advertising on whose keywords on Google. It’s a good way to see who considers whom a threat (and who’s a threat to no one). And we can also track who’s trying to associate themselves with which issues. It’s early, so expect this it get much more tangled as the campaign goes on.
Our first findings: Democratic:

Most active advertiser: Clinton.
Her campaign is advertising on Obama, Biden, and Dodd searches. Note well that she’s the only candidate advertising on the Iraq keyword. Ditto Al Gore.
Obama is advertising on Clinton and Edwards
Richardson and Kucinich are a threat to no one.

Republican:

Already more tangled:

Most active advertiser: Giuliani.
His campaign is advertising on McCain, Brownback, Tancredo. Note that he’s also advertising on “flat tax.”
McCain is advertising on Giuliani and Romney
Romney is advertising on Brownback and Giuliani
Hagel is advertising on McCain
The two Thompsons, Gingrich, Huckabee, Hagel are a threat to no one.

Note also that no one — no candidate, no brand, no news organization, no one — is advertising against “Bush” and “George W. Bush”, even negatively. Nobody wants any of that on them.

Oh, YouTube is advertising against a lot of candidates. And Comedy Central is advertising against John Kerry and Chris Dodd.

Methodology: We checked on one day, looking at variations on each candidate’s name (e.g., Hillary Clinton, Hillary, and Clinton).