It depends heavily on the page. I can personally provide an example of the "Men's rights movement" page. I used to be heavily involved in the movement with regards to parental custody. At least a decade ago, ideological moderators began applying extremely high standards for the page. Whole sections were removed and re-written. Their argument was that "these are the rules," and while technically true, the rules are diverse and opaque. They can be applied in many ways, depending on one's ideological alignment. I encourage you to take a look at the above page, and then contrast it with "Feminism." You'll notice a distinct difference to the breadth of content, the standard for citation, and especially the tone. For example, in the second paragraph on the "Men's rights movement" introduction, moderators have added an entire paragraph of criticism. You'll see no comparable criticism on the "Feminism" page in the introduction. More recently they added a "Not to be confused with the pro-feminist Men's liberation movement" at the very tippy top of the page to try to move traffic towards a feminist friendly page. No such suggestion appears on the "Feminism" page.

I could provide endless commentary on this example, but suffice it to say, it is very difficult to look at this comparison and rationally argue no moderation bias exists. Now consider that this is just one tiny corner of the site. This is happening to millions of pages. Larry Sanger, Wikipedia co-founder, has been very vocal about its political bias. I, for one, believe him.