The Right/Left divide in the US has existed for a very long time and it would take a Roots sized book to fully explore it. I'll try to make a generalized statement about it here though.
The US has been avoiding the Ideological divide for a very longtime and during that time a lot of events took place that increased the divide, yet it was rather hidden due to other circumstances. In short:
1) The Roaring 20's were the US's first real taste of Modern Wealth and Affluence
2) The Great Depression occurred, destroying that Wealth/Affluence and pretty much Capitalism as well
3) The GD caused an upsurge in Communism within the US and FDR moved the US to the Left for 2 Reasons: a) to placate the growing political power of Communism; and b) because the Left had solutions to the Crisis
4) WW2 occurred which accomplished basically 2 things: a) reinvigorated the Economy but also got the population off the Unemployment lines; b) made everyone forget about the GD
5) WW2 ended with a Bang(literally). Not only did the Horror of that War no longer hang over the heads of Americans, but the Atomic Bombs ushered in an Age(for a short time anyway) of US Dominance and Invincibility
6) As American's returned home from Overseas and settled back into Civilian life they had a completely new optimistic outlook, completely forgetting about the GD, going forward creating a Booming Economy and an upsurge in Population through Birth(the Baby Boomers).
7) The USSR develops the Bomb and the Cold War begins. The US begins to embrace "Capitalism" as a rallying cry against the "Communist" threat. Invincibility turns into Vulnerability, Optimism to Fear, Unity to Disunity(McCarthy witch hunts)
8) Baby Boomers begin to reach Voting Age and the 60's happen:
a) Vietnam War starts small but grows larger as the Decade progresses, despite opposition to it;
b) African Americans begin to press for Rights in the South bringing many Southern States into conflict with the Federal Government and other States;
c) Baby Boomers begin the Protest the war in Vietnam, especially because of the Draft
d) Women begin to press for their Rights
e) Baby Boomers begin to reject Tradition en masse, bringing birth to not only the Drug Culture and associated movements(Hippies, Beatniks for eg), but also bringing about a major reconfiguration of Religious views(Eastern Religions are embraced, Traditional Christian views rejected, New Christian sects flourish). In short, the very Culture of the US is revolutionized.
9) The Expense and Domestic turmoil caused by the Vietnam War begin to affect the Economy ending the Post War(WW2) Boom
10) The early 70's is when many events came to a head.
a) the Hippy movement comes to a disillussionary end
b) The Vietnam War is lost
c) The US President(Nixon) Resigns in Shame
d) the Oil Embargo turns a slowing Economy caused by the War into an Economic Crisis the lasts through the Decade
11) the 70's are years of malaise fueled by the defeat in Vietnam and the Economic crisis caused by the Oil Embargo, that is capitalized by the Iranian Revolution and its' subsequent Attack on the US Embassy and the Hostage Crisis.
12) Reagan comes onto the scene bringing a sense of Optimism that shakes off the Malaise seeing the end of Vietnam Defeatism, the Hostage Crisis, and the Economic Crisis
13) Reagan institutes a Counter Revolution against the 60's Culture Revolution, US Foreign Policy(re-engages War against Communism in various parts of the World), and Capitalism(which hadn't really been addressed since the GD)
14) as a consequence of 13, the US Economy Booms, Optomism returns, and the Cold War eventually comes to a head and ends
15) Bush Sr announces the New World Order and he isn't joking. Domestic Policy takes a back seat to Globalization and various other Foreign Affairs.
16) Without the Enemy of the USSR, the Domestic Ideological Divide begins to come to a head. No longer do Fear and Struggle moderate the level of Domestic discourse. It's not just a Left/Right divide either, but a whole myriad of things:
a) Of course Left/Right is one;
b) Federal/State Jurisdiction is another;
c) Race issues
d) and, really the heart of all the previous on this List, Regional issues. The Regional issues are mainly focused on the "South" and really have their roots all the way back to the Civil War, making it an especially troublesome problem for the US
17) 9/11 happens. I'm really not sure how to view 9/11, Iraq, or even Afghanistan yet. Historically anyway, I certainly have strong Opinions on what they'll Probably mean and I have stated them many times, but it's still too early to really be definitive about those subjects in context of this Post(which is trying to be neutrally Analytical, although I'm sure my opinion bleeds through in parts, but whatever). What I do think 9/11 and these other events show though, is that when faced with an external Enemy, the US can put the Domestic Ideological divide on the back burner to simmer(still is there, just quiets down). In the Longterm an External Enemy can not continue to be a way to avoid the Domestic Divide, it is a festering wound that threatens unity. The end of the Bush Jr era with its' winding down of the Iraq War and the fading emotions caused by 9/11 have reinvigorated the Domestic Divide, but this time it has taken on an ugliness that makes the Clinton witch hunts look like a walk in the park. If the US can't work things out, I suspect I'll live to see the next US Civil War.
18) Fast Forward to today and we see yet another Roaring 20's scenario end in a failure of "Capitalism" Note: "Capitalism" doesn't mean Capitalism, it is just a narrowly defined form of Capitalism that seems to get embraced by Americans from time to time and always seems to end with a tragic bang.
Summary
All y'all need to fix this shit before y'all are fucked