Obama's ratings slide

I'm told, by a liberal friend [hey I've got 'em] that Rasmussen Polls are Republican and therefore unreliable. I wouldn't trust any polls out of the networks. Gallup and Pew I don't know about. In regard to Obama's popularity Rasmussen has been showing a downward trend for a few weeks now. I looked at Gallup and any changes are minor. Now Pew is showing some dramatic changes downward. I was convinced by Rasmussen but it's nice to get some confirmation.