Why should we believe either side?

I have a real problem with negative political campaigning. Evidence suggests that it does work, but only among those who are already supportive of the message and, usually, the sender. They tend to polarise their audience, they are either for or against based on their existing predispositions. They do not convince people however, in fact they have a wholly negative impact on undecided voters and turn them away from the system generally as opposed to simply the person under attack. In order to explain why I use a video circulated by the Conservatives which is a good example of the effects of negativity.

This video asks the viewer to believe Gordon Brown is a liar and is misleading the public over his and the Conservative Party's spending plans and who will make cuts to public service spending. Those who distrust Brown and like Cameron and the Conservatives will believe it and, perhaps, their pro-Conservative voting intentions will be increased. The reverse will be the case for pro-Brown and pro-Labour supporters. That is all fine. But the problem is for the rest of the audience. The Conservatives may be pushing against an open door in terms of public opinion, around 40% indicate they would vote for them if there was an election tomorrow, the question is whether all of these people are likely to change their minds. But even if we assume 40%, it leaves some that may well be undecided or still not totally convinced. The message in the video asks them to trust the Conservatives and not trust Labour, but this can also lead to confusion. There is a further question asked: 'Who should I believe', this leads to 'Who can I trust' and importantly 'What are the motives behind this message'. If the message is deemed to be chasing votes then trust in the sender will be reduced, if also the motives of Brown are questioned then this reduces trust in him also; hence the system suffers as the audience then is seeking an alternative to both or considering abstaining as the choice becomes too difficult. Hence this strategy may actually have a negative impact beyond that which is intended!