Discredit by association

Question

Is there a word or term to describe attempting to discredit an organisation/movement/group by association of certain members involved?

For example, when discussing the Peoples Vote campaign among certain people I often here it being discredited by having Chuka Ummuna as the leader, Anna Sourby as a co-chairperson, and now Roland Rudd as supposedly being involved. All unsavoury characters no doubt about it, but bringing them up does nothing to discuss the merits of a second vote and only seeks to close down debate.

Edit
Written as a more abstract example:

• Person A and Person B are discussing whether the UK should have a second referendum on Brexit as campaigned for by the "Peoples Vote" pressure group.
• Person A is in favour of a second vote.
• Person B is against a second vote.
• Person B explains that Chuka, Anna and Roland - people who both A and B agree are unsavoury characters - are involved in the Peoples Vote group, two of whom in high profile positions. Person B uses this as a reason for why the entire Peoples Vote group is illegitimate and ultimately that because the Peoples Vote group is illegitimate that the UK should not have a second referendum on Brexit.
• Person B has not raised any points to do with the merits of the Peoples Vote objective - to have a second referendum - other than to discredit the group by association of some of the individuals involved.

I have been calling this "a sort of ad hominem" attack up till now, however, as I understood it the definition of ad hominem is to attack the claim maker so I think I am wrong in saying this. Person A in this is not making any claim. The entity being attacked is a third party - the Peoples Vote group.

This seems to be happening a lot nowadays, especially when discussing something political.

Answer 1

I think your OP needs some more context and an example sentence to be clearer. (I'm unfamiliar with the names/events you allude to). Based on what I think you mean, the logical fallacy of "tu quoque", or the appeal to hypocrisy, is a candidate:

Tu quoque (pronounced like 'too kwoh kway") (Latin for 'you also') is a form of ad hominem fallacy that occurs when it is assumed that an argument is wrong if the source making the claim has itself spoken or acted in a way inconsistent with it. The fallacy focuses on the perceived hypocrisy of the opponent rather than the merits of their argument. This is a fallacy regardless of whether you really did it or not, but it helps if you really didn't do it.

So, if a group says smoking is bad and you try to discredit the group by pointing out that group member Mr. X is a smoker; you are appealing to hypocrisy.

PS - If you're saying Mr. X is simply an "unsavoury character" (not necessarily a smoker)—and there's no hypocrisy involved—it's a more general case of ad hominem.

^{[Source - rationalwiki.org]}

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After the OP's edit:

I wouldn't exclude it from the purview of ad hominem even if they are discrediting the group and not the person. That being said, I can't think of anything that precisely describes your situation, but it has elements of the following:

Reactive devaluation

In a discussion, one’s opinion about a certain proposal is influenced by his opinion of the person suggested it. If we consider the person making the offer as an antagonist, we tend to value the proposal as being less interesting than if it was made by someone else. This cognitive bias is known as reactive devaluation and can affect your decisions as well as block your influence in conversations.

EXAMPLE

A sidewalk survey was made in 1986 where pedestrians were asked if they would support a nuclear arms reduction plan. 90% of the participants answered they would if they had been told that President Ronald Reagan made the proposal but this number dropped to 44% when they had been told that Mikhail Gorbachev made the proposal.

^{[Source - thedecisionlab.com]}

Bear in mind, this is a cognitive bias, not a fallacy. Which means it applies only when someone actually believes that the idea is bad because Chuka/Anna/Roland (or their group) suggested it; not when they're just using that argument to win a debate, despite knowing better.

Fallacy of composition

The fallacy of composition arises when one infers that something is true of the whole from the fact that it is true of some part of the whole (or even of every proper part).

^{[Source - Wikipedia]}

Chuka, Anna and Roland are untrustworthy. So, the whole group is untrustworthy.

Answer 2

The word you seek is “tarnish”.

See also “malign”, “bring into disrepute”, “besmear”, “taint” besmirch” vilify”.