Okay, this has actually occurred in other threads BUT I'm adding some rules to it.
1. The date it must occur after is 1950. So this isn't something with Teddy.
2. At first, it must completely wipe the laize-fair and religious fundamentalist parts out of existence at least briefly politically speaking. Miminum time for that is 8 years.
Those are my rules. Now, this is probably ASB, but I've been proven wrong many times here. I hope to be again.
OK. Leftist-wank, but here goes:
Nixon decides to fight it out during Watergate. He resists any calls to resign, and works the phones HARD to line enough Congressional support to resist impeachment through threats/blackmail/bribes/whatever. He also COMPLETELY panders to the extreme right-wingers by going after Draft Dodgers much hard than IOTL, fighting Roe Vs Wade with judicial appointments, scaling back social spending, and doubling down on Vietnam.
All of this keeps his administration creaking along until 1976. With Vietnam still going on, Kennedy decides to run and wins the Democratic nomination. By now, the country is a mess. Mass protests over Vietnam, growing alienation overseas, a resurgent USSR and grinding recession virtually guarantee a Kennedy victory.
With that, the Republican nomination is a mess. No one 'serious' in the party wants the nod because the Republicans aren't going to win. However, Nixon's pandering has inflamed the right-wing base of the party; call it the Tea Party 30 years early. For a while, the nomination looks to be Reagan's, but he is dumped eventually in favor of Strom Thurmond, a better avatar for this proto-tea party.
Needless to say, Kennedy wins easily. After inauguration day, new AG goes in, turns up lots of evidence against Nixon, and some who supported him. This demonizes the mainstream Republicans who worked to keep him in power. Meanwhile, Kennedy ends the war, and through stimulus spending stabalizes the economy. Vietnam falls to the communists, and Kennedy pardons the draft dodgers - and Nixon, though not Nixon's political allies and cronies.
Kennedy's liberal domestic politics, 'socialistic' tendencies and the quiet 'mainstream' Republicans cause the Tea Party - under Strom Thurmond and Jesse Helms - to erupt in a viturpous orgy of hate. Using the new cable channels and talk radio, the Republicans whip themselves up into a frenzy of half-baked conservative ideas that go from the unlikely (return to the gold standard) to the impossible (eliminate income tax) to the bizarre (Kennedy is a KGB agent) to the downright scary (adopt Aparteid in the United States).
All of this kills the Republican Party. Despite a lackluster economy, no foreign triumphs and some questions about his personal life, Kennedy easily wins re-election in 1980. The mainstream is unhappy with him, but downright terrified of Strom & Co.
As the 1980s progress, the Democrats enjoy uninterrupted control of the legislature and executive branches. Strom and Jesse's longevity account for this. However, that control does not equal legislation. Many 'conservative' Democrats come into the party. Intra-caucus fights in the House and Senate become legendary as Democrats fight amoung themselves.
Eventually, in 1996, a conservative Democratic Senator from Arkansas, Bill Clinton, disgusted at the election of Paul Simon as POTUS, forms a new party - the Constitutionalists. The Constitutionalists actually attract the fiscal conservatives and libertarians. The Constitutionalists platform differs from the democrats on economic, foreign policy and domestic spending issues; it is socially liberal and open to all. In 2000, Clinton beats Simon to become POTUS...
There. The Republicans are not dead, but win no electoral votes. There are still two parties, but the overt religion is gone, at least at the national level. After more than two decades of Deomcratic rule, social programs are entrenched and (for now) accepted. The USA is still capitalist, so I guess I missed on that.
Mike Turcotte