Here's one mixed bag of possible explanations. Some are competing, some are complementary and any one would require pages of elaboration or proof:

1. The New Deal/Great Society coalition disintegrated, replaced by a GOP coalition led by social conservatives and corporations.

2. Money replaced organizing in terms of importance for winning elections. Dems abandon the working class in pursuit of campaign cash.

3. Communism collapsed, undermining the motivation to pay attention to equality or a safety net.

4. The long-time generational pendulum in U.S. politics swung right, leaving more liberal Dems with a narrower base.

5. The Military-Industrial Complex, tax cuts and the resulting deficit spending made ambitious domestic programs more difficult.

6. U.S. workers -- a key part of the progressive Dem base -- lost their power as WWII industrial powers recovered, industrial centers emerged in the 3rd World, and global movement of good and services increased.

7. Massive increases in the prison population disenfranchised millions of African Americans. The threat of deportation and lack of a vote detered political participation by millions of the poorest workers (illegal immigrants), thus knocking the political legs out from under any attempt to promote progressive economic policies.

8. Television eroded social networks that facilitate grassroots politics in the working class, and made it difficult for more radical voices to be heard or accepted.

Whistler Blue: Excellent points.

Interesting list. Here's a question: of the eight items, which factors leading to the weakening of the Dem party did the leadership of the Dem party actually oppose? Growth of the Military Industrial Complex? Sending jobs overseas? The huge expansion of the prison system? A stronger role for money in politics?

Nah, didn't think so.