The New York Times offers this report, which begins: “Senator John Kerry’s advertising campaign is so closely complemented by those of two major liberal groups running commercials against President Bush that Republicans are accusing the Democrats of trying to evade campaign finance laws.”
The most important snippet from the article:

Mr. Goldstein said it was entirely possible to devise advertising strategies that correlate this closely without speaking directly by monitoring purchases, highlighting the difficulty such cases will have before regulators or judges.