It seems as though almost as many contradictory polling results as mud has been slung around in the last few weeks of American electioneering in the attempts to boost one candidate by smearing and vilifying the other.

This interesting recent article from Pollster.com contrasts and compares the Gallup Daily series with Rasmussen Reports daily tracking poll over the same six week period.

it concludes....

'First, there has been far more stability than change in the national Obama-Clinton vote preference since Super Tuesday, and that includes the period of last ten days. To the extent that we have seen real changes, they are barely bigger than what we might expect by chance alone.

Second, if you look closely, you will notice that the seemingly odd divergence between Gallup and Rasmussen since the Wright story broke is really not that unusual. It is comparable to similar separations in the trend lines that occurred around February 13 and February 29. Random variation will do that.

Third, and probably most important, it is far too easy to look at these rolling average tracking surveys and see compelling narratives and spin interesting theories from what is often little more than random noise.'

Distinct from the tracking polls are the 'panel surveys' which claim to be very useful after some major event.
CBS used them following debates/speeches etc to track which people opposed beforehand changed their minds afterwards.

On March 20th following Obama's speech they concluded -

'We found no sizeable overall change in Obama's rating after his speech, though there was a lot of internal movement. Fifteen percent of those who had a favorable view of Obama before his speech did not maintain it afterwards in the second interview. Twenty-three percent of those whose opinions were unfavorable also changed. Most of those who changed their opinions went into the "undecided" category, suggesting that the speech had given them something new to think about. Forty-two percent of those who had been undecided before the speech said they had an opinion after - but they were almost evenly divided between positive and negative opinions. The impact was greater with some groups than with others - women were more likely than men to move into the "undecided" category after the speech. Obama lost a little ground with independents, and even with Democrats. Republicans had been unfavorable before, and they remained that way.'

I present these figures not as a voter caught up in the fervour of campaigning but as a European with a wish to see a more liberal America merely observing some sophistry from the sidelines.