In my 16 years of business, I have witnessed several instances where people have broken down to nearly crying after a meeting where a business "argument" ensued (it happens to men, but I mostly have witnessed it with women). Essentially there was a matter of opinion on what to do, how to budget, or why a schedule was missed, and the discussion became heated.
In my effort to make them feel better I tell them that the discussion was "business, not personal" al la Michael Corlione. I tell them that it wasn't worth getting upset over a difference of business opinion and to move along to other things that are more important, even if they are convinced they are right.
The same can't be said of politics. You never hear "it's politics, not personal". And because politics touches so many aspects of our lives, it is pretty much impossible to "move on to other things".
I have read on blogs about friendships becoming frayed over political discourse and I think I have lost one friend over an argument over F911. Political differences are seen as character flaws and unlike business discussions, it is nearly impossible to move along to other things once an argument has ensued. If there is no political common ground between two people, can a friendship really exist?
That's not to say that I don't have friends that I have political disagreements with, or even political arguments with other bloggers I like. For example I recently posted some differences of opinion on Jim Carson's Media Diversity Quiz. Jim and I have known each other 18 years and Jim drives straight down the center of the road and I am over on the right shoulder. We know we're are going to disagree on a lot of issues, but it doesn't change our long-standing friendship. But as I noted in the comment thread, we do agree on about two-thirds of the issues. Would the friendship be different if we agreed on 0%?
Even in marriages where there is one democrat and one republican, my experience is that both parties have to be fairly moderate if the marriage is going to work. Like the democrat VC (Ed: that's Venture Capitalist, not Viet Cong) I know who is married to a republican, the fact that she is a VC means she is pro-business, anti-regulation, and pro free-trade - not exactly left-wing positions. Essentially she is democrat due to a single issue, which is true of most democrats I know (the single issue might vary, but is seems the democratic party is largely a collection of single-issue voters).
This, of course, assumes that politics matters to both people. There are guys who only care about sports, romantic couples who are only interested in the physical, and people who are oblivious about politics in general, but assuming that both people hold deep political views, can they be friends if they are polar opposites? My experience says no. There has to be some common ground or there is no basis for the friendship.