An idea doesn't become wrong because no successful party supports it. It doesn't become right because a small, unsuccessful party does. We ought to be able to judge a proposal without treating the polling of whoever suggested it as the answer.
That sounds obvious until you watch how quickly an argument about a policy becomes an argument about whether its supporters could win an election.
Consent still matters
There is a fair objection. Politics involves persuading people. A party can't govern on the strength of being privately convinced that it knows best. If it can't explain its programme in a way voters accept, that is a political failure, even if parts of the programme are sound.
A party that responds by calling the public stupid has learnt very little. Voters are entitled to disagree, and policies involve values and priorities as well as factual claims.
But winning attention and governing competently require different skills. A confident promise or a memorable opponent can attract an audience much faster than a careful explanation of trade-offs. Patient administrative work rarely produces a clip worth sharing.
Good communicators can be good governors. They can also be dreadful ones. The performance doesn't answer the question either way.
Give the proposal a hearing
A smaller party may have an excellent diagnosis that few people hear. It may also have a terrible one. Its obscurity gives us little reason to decide in advance.
The same applies to a popular party. Its success tells us that it has persuaded people to support it. That matters enormously in a democracy, but it doesn't establish that every factual claim it makes is true or every proposed remedy will work.
I would rather ask what a policy is meant to achieve, how it would do it and what it would cost. If a proposal survives those questions, the absence of a political home is a problem for our politics. It isn't a reason to dismiss the proposal.