rumor campaign
A concentrated and prolonged effort to damage or ruin someone's or some group's reputation through the perpetuation of rumors, innuendos, and/or falsities, generally as a means of persuading a large number of people against him, her, or them. Primarily heard in US. The local sheriff's slim lead heading into the election quickly evaporated after he found himself at the wrong end of a vicious rumor campaign. I think we need to start a rumor campaign to slow down the competitor's growth in the market, but no one can know that it's coming from us!