This piece by Josh Marshall tracks pretty well with how I feel about the changes to politics that've taken place and why it forces moderate Democrats like himself to be partisans.
I live in Manhattan and have a certain perspective on the country. Folks in Oklahoma or evangelicals in South Carolina have a different one. And that's fine. It's their country too. What I think is that a certain political movement has taken over the country -- call it movement conservatism in it's late, degraded form -- and wants to govern it by all or nothing rules.
One of the things I've always cherished about America is that the country was founded on the principle of institutionalized disagreement. The founders recognized that only vibrant debate would bring consensus and built mechanisms into our representative government to govern the rules of that debate while guarding against excesses (balance of powers, veto, etc.) For the most part that system has worked well over the course of the history of this country (the big exception being the civil war.)
Like Josh I recognize that their are a whole heck of a lot of Americans who have different ideas and different perspectives about how things should be run. I'm not always going to get my way and that's okay. The genius of the system is that the protections on speech mean that I have the rest of my lifetime to try and convince those that disagree to come around to my way of things. If I can convince enough people then we can change the laws to reflect our point of view.
For this system to work a vast majority of Americans have to "agree to disagree" and do so within the system. Should there be a signifigant minority unwilling to face the results of government by consensus you could have a violent rebellion of those in the minority. A "war between the states."
But the system has another Achilles' Heel that's become apparent over the course of the last several years: apathy. As fewer and fewer people participate in the system then the old truism to politics that elections are decided by swing voters will die and only the most motivated, who tend to be most extreme in their views, will get to the polls and make determinations on our governance. A political party that recognized this and was unscrupulous enough to take advantage of it might be able to game the system. Once they'd reach that point and since they'd already decided the system of institutionalized disagreement wasn't working to their advantage they'd take steps to further breakdown those sytematic roadblocks to extremism in order to work to permanently consolidate their worldview.
Josh points to the Bush administration as the beginning of the death of the Democratic moderate. I would personally put it back a little bit further. The impeachment of Bill Clinton was the moment when it became clear to me that these people weren't just cute little conservatives that I would disagree with on a few issues and actually dangerous radicals who had no use for our system of governance.
Like Josh I'd like to get back to a point when governmental policy was actually determined by debate on the issues rather than today's political tribalism. It's clear that this will never happen while the radicals are in charge and I'm not so sure about afterwords either. But for our government to work as intended we have to have everybody; left, right and center engaged in the process and willing to play by the rules.