Polling firms uses different standards (like the questionable Zogby internet polls or the current LA Times un-weighted poll), which makes it difficult to compare them or decide which will be most reflective of the voters opinions.
I think the Rasmussen daily polls (really three day polls released each day) maybe one of the most useful. Every day he uses the same methodology, polls the same number of voters, applies the same weighting. Thus although his absolute numbers may or may not be correct, any real change in the voters preferences from Bush to Kerry or vice versa should be very reliably detected by him.
The amazing thing is that since early March, there has been no significant change, its been a statistical dead heat: on any given day Bush and Kerry bounce around 44-47%. There is no evidence that any of the advertising, campaigning, or Iraq events have substantially changed either candidate’s support. For June Bush has been ahead on 4 days, Kerry on 3, and both tied on 5 days.
This poll is updated every day at noon:
http://www.rasmussenreports.com/Presidential_Tracking_Poll.htm
