There are a lot of internal discussion groups - it is just an internal Google groups thing so anyone can create a group about anything. Often they are purely work related (e.g. groups for specific tools/services/teams that people can join), but there are some that are not work related.
I'd say that these sort of political debates are the extreme outliers. I've never encountered any significant political debate/argument during my 6 years at Google, but then I don't go looking for it on the groups. From my experience people at google are well-mannered, polite and respectful; disagreement of course occurs but is usually handled professionally without much fuss.
What does happen though is that although we do have an online tool to vote up questions for the TGIF meetings, you get "regulars" who ask in-person questions at TGIF in Mountain View. These people are often fine, but there are a couple who clearly have their own political/moral axe to grind. I find these people the most irritating, since unlike the questions which have been voted up to the top on the online tool, they don't need votes to ask their questions. This can give their opinions -minority or majority held- more apparent credibility than they might otherwise deserve if it had been left to the online voting only.
There is also a lot of focus from certain groups on purely US related things - e.g. asks about allowing people to volunteer if they are conservative or democratic in the HR system. These blinkered people asking these in-person questions in California don't realise that they live in one country, and that the political parties in that country aren't the same as the ones in others (most countries are not a two party polatised system like it is in the US)
TL;Dr there are jerks if you go looking for them but 99.9% of the time yiu just do your job and no one wants to know or cares what your political or moral views are.