Senator Richard Lugar's loss in his Republican primary in Indiana this week continues a trend which has been evident for at least the past two decades in American politics -- the matchup of our political divide with our ideological divide. What this means for the future is really anyone's guess, other than that "R" or "D" after a politician's name is a lot easier to "read" these days.

The partisan divide -- Republicans and Democrats -- has been in place in American politics since the Civil War. For the past century and a half, there have been two main parties in this country. Third parties have popped up from time to time and had measured success, but at the end of the cycle, they usually get absorbed by one of the major parties -- or the third party just fades away into the woodwork.

The ideological divide is a tougher one to trace throughout history, because "conservative" versus "liberal" doesn't fully encompass the different sides of issues these two groups have, at times, taken. In fact, even the use of the word "liberal" is on the wane (mostly being replaced with the re-tread "progressive"), even though the ideals haven't changed all that much. But branding issues aside, this is the true division of our country's politics. What is odd about our current situation is how closely the parties align along this fracture.

"Conservative Democrat" and "Liberal Republican" didn't used to be oxymorons, to put it another way. The parties themselves had "wings" of conservatives, moderates, and liberals. Sometimes these divisions were geographical, sometimes more ideological. Even at the beginning of our history, the parties had "Northern" and "Southern" wings -- which held true right up to the Civil War itself. Since the end of the Reconstruction era, the two parties still had differing factions within them. Republicans and Democrats today simply can't just assume that their parties' stances today were the same throughout the past century.

Democrats, after all, were the ones to filibuster the Civil Rights Act. Democratic senator Robert Byrd was a former member of the KKK. Martin Luther King wasn't sure which party he should approach to back civil rights legislation, because there were both Democrats and Republicans open to the idea -- and there were also Democrats and Republicans who were violently against the concept.

What has changed now, though, is this party/ideological alignment. Since roughly the time of Newt Gingrich's ascension in the House, the parties have been getting more and more like-minded. Some might trace it back even further, perhaps even to the Civil Rights Act itself. But it is undeniable that the two parties have become "purer" ideologically, and Lugar's loss is just one step along this path.

Liberal and moderate Republicans are a dying breed. There just aren't that many left, at least on the national level. Conservative or moderate Democrats are more prevalent, but are also on the wane. The "Blue Dogs" of the Democratic Party were decimated in the 2010 elections, moving the party (in the midst of its electoral "shellacking") further to the left. Republicans now have the Tea Party to enforce purity, and as a result the moderates are disappearing, one by one.

I draw no conclusions, personally. I don't know, to put it another way, whether this is a good thing or a bad thing for American politics, and for America. Party-line votes have become the norm, and not the exception. The parties themselves have almost perfectly lined up along the same old ideological divide which has always been there. Republicans, in particular, seem almost frightened now of the word "compromise" -- one of the key factors in Lugar's loss.

This is a very odd period in American politics, but the difference isn't that we're "more polarized" it's simply that the parties themselves are "perfectly polarized." The polarization has always been around, but now it's just a lot easier to see which side someone is on. Soon, for any national politician, all you'll have to do is look at the "D" or the "R." We're almost there now, in fact.

-- Chris Weigant

Follow Chris on Twitter: @ChrisWeigant