Well there are a couple of reasons for it.
FIRST, party IDs are self-reported. So they tend to change -- if Republicans are popular more people will self-identify as Republicans, regardless of whether or not they register by party in a particular state. So it makes sense that there would be more self-reported Dems if polls show Obama ahead. I grant that it's weird and seems counter-intuitive.
SECOND, there are more "registered" Dems than GOPers in the country. As far as I know, this has always been the case. That may or may not have anything to do w/the party people tell pollsters they belong to. There are a bunch of reasons for the Dem registration advantage -- still a few conservative Southern Dems who now vote exclusively Republican, alienated Northern Catholics who still ID as Dems but often vote GOP, etc.
ALSO, we've seen a growth in the number of Independents, voters who often, as seems likely in November, skew Republican.
This seems correct.
I suspect in addition that Rasmussen has also had to change their party affiliation/turnout model, which is also dynamic and based on, I believe, a separate poll.
So a parallel change in different Rasmussen polls could magnify a bump. This could easily work either way.
It amazes me how ignorant and uninformed you guys are. What is defeating you is your ignorance not the Democrats.
The fact is THERE ARE MORE REPUBLICANS THAN DEMOCRATS in the country according to party affiliation exceeding Democrats by 4%. 37.6% (R) 33.3%(D) 29.2%(other). This situation has been trending Republican for a while now for the Republicans for over a year. This information comes from Rassmussen and you guys have not been aware of this for over a year. Taking these trends into account the Democrat polls make no sense.
http://www.rasmussenreports.com/public_content/politics/mood_of_america/partisan_trends