Many opinion pollers do badly when it comes to predicting elections. This is mainly because they let their respondents self-select them into their polls. So what, who cares? The polls make for some good entertainment and easily fill the talk-shows on television. If everyone knows they cannot be trusted, why care?
We should care. In the Dutch electoral system - with poportional respresentation - every vote counts. If only a small percentage of voters lets their vote depend on the polls of the election result, this can result in shifts of several seats in parliament. It is unclear how many voters decide how to vote based on the opinion polls, but it is a fact that there are many voters who consider voting for two or more parties, and many who do vote strategically. The Dutch Parliamentary Election Study (DPES) in 2006 found that 18% of voters indicated that they let their vote be influenced by the election polls. This amounts to a total of 27 parliamentary seats: almost the number of seats of the largest party in the current parliament.
As long as voters choose strategically in different ways this may not matter. If someone votes strategically to make sure a new government has the the greens in it, but someone else votes strategically for labour to make sure his or her favourite candidate becomes prime mininster, the net effect of strategical voting might be zero or very small. There is evidence however, that this is not the case. People like to vote for winners. This is called the bandwagon effect. Whenever labour does well in the (biased) opinion polls, more voters will consider voting for them. This may in the end lead to the fact political parties (and pollers) have a lot of interest to do well in polls. In fact, it may be tempting to publish fraudulent polls. This seems to be increasingly common in the United States, where they call them "push polls" . Publish fraudulent polls on purpose to make public opinion shift in your favor.
So, what to do about it? First, I think it would be fair not to publish any opinion polls some time before election day, as is done in France for example (albeit only for two days). Second, journalists and newsreaders should be very critical towards opinion polls, and only publish them when some basic quality criteria have been assessed and met. The Dutch Organisation on Survey Research has taken the initiative to develop a checklist for journalists. I will put it online soon.