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