Politics has ALWAYS been divisive. Hell, some of our founders killed each other in DUELS. There were fistfights on the floor of congress.
However, I do think we're seeing a period where both parties AND THEIR VOTERS are moving further and further from center. I have three main causes.
1) Computer Aided Partisan Gerrymandering. There's nothing new about partisan gerrymandering. What's different is that computers and demographic data mean that it's now possible for consultants to draw hyperaccurate maps to achieve VERY specific election outcomes. General elections as a result are safer than ever. Incumbents who lose almost ALWAYS do so in their primaries. SCOTUS recently passed on addressing this, and I can't fault their legal logic for doing so. But this is a BIG problem, and both parties do it.
Safe general elections and threatening primaries means the average politician only gets threatened from their extreme flank. They don't have to worry about a centrist threat. So they become more extreme in order to inocculate themselves from the primary challengers.
2) McCain-Feingold and other Campaign Finance Reform. Anytime someone talks to you about getting money out of politics, you should punch them in the mouth. Every campaign finance reform has resulted in making incumbents safer, and the biggest was McCain-Feingold (MF).
MF's main contribution was to eliminate Soft Money, which was people donating to parties instead of candidates, and then the parties disbursing that money. Because it was uncapped, unlike Hard Money (which went to candidates), campaign reform advocates claimed it was a source of corruption. This was a lie.
Soft Money almost always was spent by the party on behalf of CHALLENGERS, not incumbents. Incumbents have a built in money and fame advantage and don't usually need help. So parties usually spent money on challengers. Soft Money in other words was making elections MORE COMPETITIVE. Which is why incumbents HATED soft money.
3) Modern Media. Let's not fool ourselves, media in America has never been as good as people some times think. Yellow Journalism and using newspapers for personal attacks of vendettas is as old as the very existence of the printing press. Maybe older.
But what has changed is that somewhere along the line, people developed faith in journalists. A misplaced faith. And the industry has fragmented and decayed since then. Now everyone picks their news source to confirm their own biases. Liberals only listen to liberally biased sources, conservatives only read conservatively biased sources.
And across all sources, journalistic responsibility is gone. The days of every article requiring two or more RELIABLE sources to go to print are gone. Now, even the slightest rumor goes straight to air. Being FIRST is more important than being RIGHT. And retractions are seldom issued, and never prominently. It's not uncommon for opinion to masquerade as journalism, and too many viewers can't tell where one ends and the other begins.
Because people choose their own sources to meet their bias, most people live in echo chambers where they never even hear opposing ideas presented in their original form...instead they at best will hear opposing ideas presented in a slanted derogatory form that ensures no one can find common ground.