A smear campaign, smear tactic or simply smear is a metaphor for activity that can harm an individual or group's reputation by conflation with a stigmatized group. Sometimes smear is used more generally to include any reputation-damaging activity, including such colloquialisms as mud slinging.

Smear campaign
Smear Campaign
Smear campaign
Author of book on Scientology tells of her 8 years of torment
Some in congress seek inquiries on cult activities
Eye TV // Cultists cornered
Literary review // A profit without honor
L.A. newspaper alleges Scientology plot
THE SENATE MATTERS OF PUBLIC INTEREST
Van Jones: A Welcome Return for a Green Jobs Visionary
We mustn't allow Muslims in public life to be silenced
A (blood) smear campaign.
Generic smear campaign