A change in voting behaviour must cause the parties to alter their policies and campaigns; and it is the minority of voters who are prepared to change their vote who really influence the party policy.
In addition, a more proportional voting system makes the ability to form coalitions a marketable asset. The parties need to emphasise areas where they can compromise and co-operate, not only those where they stand clearly different from their fellows.
One of the most powerful arguments for PR is that it favours politicians and parties who look for common ground. Where different parties find agreement it is far more likely that there is an underlying rational truth to support the policy than a mere coincidence of blind prejudice. Thus PR favours the rational and the enduring over the emotional and fickle.
Critics will say that when tough radical policies are really necessary PR will not deliver politicians tough and powerful enough to see them through. This is an illusion created by the present system, which does not enable people who are both tough and rational to rise high in politics. Such people cannot gain sufficient support within one party, because rational people tend to repel some of their more emotional colleagues. And first- past-the-post cuts them off from support from within any other party.