The policy has ALWAYS been split. Hell, some of our founders killed themselves in DUELS. There were fist fights on the floor of the congress.
However, I think we are witnessing a period when both parties AND THEIR ELECTORS are moving further and further away from the center. I have three main causes.
1) Computer-aided supporter Gerrymandering. There is nothing new in partisan gerrymandering. The difference is that computers and demographics mean that it is now possible for consultants to draw hyper-accurate maps to get VERY specific election results. General elections are therefore safer than ever. Holders who almost always lose do so during their primaries. SCOTUS has recently passed, and I can not blame the legal logic. But it's a big problem, and both parties are doing it.
Secure general elections and threatening primaries mean that the average politician is only threatened by his extreme flank. They do not have to worry about a centrist threat. Thus, they become more extreme in order to invoke the main competitors.
2) McCain-Feingold and other campaign finance reforms. Whenever someone talks to you about withdrawing money from politics, you should hit him in the mouth. Each reform of campaign funding has made the incumbents more secure, and the most important has been McCain-Feingold (MF).
MF's main contribution was to eliminate Soft Money, which consisted of making donations to parties rather than to candidates, and then to the parties that were paying this money. Because it lacked a hood, unlike Hard Money (intended for candidates), advocates of campaign reform claimed it was a source of corruption. It was a lie.
Soft money has almost always been spent by the party on behalf of CHALLENGERS, not holders. The incumbents have an advantage in terms of money and notoriety and do not usually need help. The parties usually spent money for their challengers. Soft Money, in other words, made elections MORE COMPETITIVE. That is why the companies in place hated the soft money.
3) modern media. Let's face it, the media in America have never been as good as we sometimes think. Yellow journalism and the use of newspapers to personally attack vendettas are as old as the very existence of the printing press. Maybe older.
But what has changed is that at one point, people have developed their trust in journalists. A misplaced faith. And the industry has been fragmented and rundown since then. Now, everyone chooses his source of information to confirm his own prejudices. Liberals only listen to liberal sources, Conservatives only read conservative sources.
And in all sources, journalistic responsibility has disappeared. The time of each article requiring two or more RELIABLE sources to be printed is over. Now, even the slightest rumor goes straight to the air. Being FIRST is more important than being TRUE. And retractions are rarely made, and never in evidence. It is not uncommon for opinion to pose as journalism and too many viewers do not know where one ends or the other ends.
Because people choose their own sources to reflect their biases, most people live in echo chambers where they do not even hear the opposite ideas presented in their original form … instead from this, they will at best hear opposing ideas presented in an inclined pejorative form that does not guarantee anybody can find a ground of understanding.