Actually this would work nicely if the body that makes this survey doesn't work for any of the candidates, but either has independent votes or is funded by the voters. It would then be in their best interest to show the voters all the evidence, rather than "all the true evidence that serves my candidate".

In other words, if you want to intervene in politics as a rational agent, you shouldn't work for any party: you should work for the public at large! Which brings us to the following question: what is the necessity, nay, the justification for parties existing in this day and age? Aren't there better alternatives in making governments be the faithful servants of popular will, rather than, say, of their own existence or of the interests of a particular group of people?