Jonathan Haidt and Marc Hetherington provide a short history of political polarization:
Political scientists cite many reasons for the dip in the middle, including shifts in the coalitions that composed each party, the shared experiences of war and economic calamity and very low levels of immigration, which allowed a stronger sense of national identity to form. E pluribus unum takes time.
But things started to change in the 1960s and 1970s, as the Democrats became the party of civil rights and the Republicans forged an alliance with the religious right. By the 1980s, the two parties were well on their way to ideological purification: liberals and more recently moderates no longer felt at home among congressional Republicans, while conservatives felt unwelcome among congressional Democrats. The trend has been steady, continuing right up through the imminent departure from the Senate of Olympia Snowe, one of the last remaining moderate Republicans. On the bright side: it is mathematically impossible for congress to get much more polarized.