I want pollsters, for example, to provide the party with accurate data. A pollster that is forced to take an ideological position for or against the party is more vulnerable to influence and bias.
I'm not seeing it.
Pollsters who are working as consultants for a campaign are doing a job. They're getting paid to provide their client with whatever it is their client is paying them for. In theory, accurate data, but anyone getting paid to provide a service is vulnerable to the impulse to give the customer a favorable impression of the service they're paying for. This is unavoidable.
What the DNC is saying is that pollsters who take on consultancy jobs for primarying incumbents will find the rest of their potential client list evaporating - regardless of the quality of the data they provide. It's not a question of taking an ideological position about their data, but about their choice of clients.
Personally, I'd be more inclined to trust a non-ideological, mercenary consultant in this scenario. Someone who switches parties, from race to race, based purely on who will pay him the most for his services, is more likely to give good quality service than someone who sticks to a particular client or client list for ideological reasons.
But whatever. The DNC is saying, "whether you're selling what your clients need to know, or selling them what they want to hear, if you don't sell it to our incumbents now, you won't sell it to our incumbents ever".
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Side note: Does the DNC also blacklist consultants who have worked for GOP candidates in the past? Or is competing with them in a general election different from competing in a primary?