Rasmussen Vs Daily Kos/Research 2000 tracking polls
Rasmussen has McCain ahead by 3%
Research 2000 has Obama ahead by 2%
Why do they differ ? The difference is not statistically significant, but it is interesting.
The reason is that the internals don't explain the difference
Rasmussen reports says "McCain is supported by 90% of Republicans and has a six-point edge among unaffiliated voters. Eighty-two percent (82%) of Democrats say they’ll vote for Obama.
DailyKos says that
McCain is supported by 91% of Republicans and has a five point edge among independents and Obama is supported by 83% of Democrats.
Given the limited information from Rasmussen reports, my best guess is that the internals explain less than 1% of the difference and the rest is due to different estimates of the numbers of Republicans, Democrats and Independents.
To me this is interesting news, because Rasmussen uses a three month average for this number, while research2000 uses the same recent data as for the presidential preferences.
Thus there is rather less than no evidence of a recent increase in Republican affiliation.