Why have the major parties been polarizing since the 1970s?

[buritobr]

Since the 1970s, the Republicans are moving to the right, both in economic and social issues. The Democrats are not moving to the left in economic issues, but they are moving to the left in social issues. So, American partisan politics is becoming more polarized from the 1970s to the 2010s.

However, the public opinion is not polarizing. The percentages of liberals, moderates and conservatives remained stable from the 1970s to the mid-2000s, and only after that the percentage of liberals and conservatives increased a little bit and the percentage of moderates decreased a little bit.

If the voters are not becoming very polarized, why did the parties become so polarized?

[Del Tachi]

The 24-hour cable news cycle has replaced debate with punditry, and political discourse has become much more divisive because of it. 

[Cassius]

I'd put it down to a few things:

1. The nationalisation of politics: Back in the 1970's, politics was rather regional, in the sense that you could have reasonably conservative southern Democrats and fairly 'liberal' (in the European sense of the term) northern (especially north-eastern) Republicans. Since the 1970's, it seems as if politics has become rather more nationalised, with candidates increasingly reflecting the average view of their party no matter where they are running. This is not to say there is no variation, for example, a Massachusetts Republican will still be more liberal (at least in terms of personal morality) than the average Republican. However, even in the leftier states, the Republican party seems to be rather right-wing (e.g. California).

2. 'Culture Wars': Now, this term is rather overused, but nevertheless, it serves well as a basic moniker for the collection of morality-based issues (abortion, gay rights, the role of religion etc) that became more and more prominent as the 70's, 80's and 90's progressed. Unlike 'economic' issues, such as the level of taxation and approaches to monetary policy, movements on moral issues are rather 'final', in the sense that, often, if one side wins out then it represents a total defeat for the other side. Therefore, politicians (and especially 'activists') are less likely to give ground on issues such as these, as 'their people' to a certain extent, have more to loose. For example, you might be forced to swallow spending cuts in one budget, but you'll have a potential opportunity to reverse those cuts when the next budget comes round. The same, generally, cannot be said for moral issues, such as gay marriage.

3. Increasing power of 'activists': This largely stems from the full introduction of Presidential Primaries in the 1970's. Whilst it would be silly to suggest that activists played no role politically beforehand (I mean look at the abolitionist and temperance movements), the selection of Presidential candidates entirely by primaries (and caucuses) has substantially weakened the power of party bosses and 'establishments' in choosing presidential candidates. A perfect example of this is Mitt Romney's selection as Republican Presidential candidate in 2012. I have no doubt that if that had been conducted under the system that endured into the 1960's, he would have been rather easily selected. Instead, because large sections of the base viewed him as being too moderate, he was forced to fight a fiendishly difficult primary battle against more right-wing opponents, pushing him further to the right. A similar case could, potentially, be made for Carter's primary fight against Kennedy, which forced him to adopt more 'left-wing' rhetoric than he had in 1976.

These are just a few reasons (I'd argue that Del Tachi's point was applicable also), but hopefully this goes a little way to answering your question.

[Heimdal]

The last 20-30 years, Americans have to an increasing extent sorted themselves out along ideological lines. People with progressive views can move to the liberal areas around the coasts, and the same goes for conservatives. That means that an increasing amount of the people live in areas where almost everyone votes the same way. You can witness this in the Dallas suburbs, and a lot of neighborhoods in New York City. The people representing these areas in Congress have no incentive to seek the middle ground, since the only threat to them is being primaried from the left (in case of the Democrats) or from the right (for the Republicans). If you live in these areas, you are rarely subjected to points of view that differ substantially from your own. This effect is reinforced by the media. In the 1950s and 1960s everyone viewed the exact same television programs. Today you don’t have to do that. You can watch Fox or MSNBC instead. This creates a self-sustaining echo chamber.

In the same period the parties themselves have sorted themselves out politically. There are still some Republican holdouts in the Northeast, but they are incredibly weak. The same can be said about the Democrats in large areas of the South. That means that an Republican president can’t rely on support from conservative Southern Democrats, just as a Democratic President can’t rely on moderate and liberal Republicans from the North. There simply aren’t anyone there. This marks an important change in just a few decades. LBJs Civil Rights legislation wouldn’t have passed without the support of people like Everett Dirksen (R, Illinois), and Reagan couldn’t have passed his tax cuts without the support of conservative Democrats.

[Amenhotep Bakari-Sellers]

The moderates have been decreasing in strength since then. No major wars like Communism or the War on Terror have united the party. Influence of 3rd party groups like Koch and Mike Moore Occupy Wallstreet has entrenched the parties due to the income divide.

[smoltchanov]

Well, i generally agree with almost all said above. But what do we have a result:

1. Much less interesting general elections. Only about 10-15% of seats are, usually, competitive in GE (in House, slightly better percentage - statewide). Wave years are exception, but otherwise - you can assign seats simply on basis ov PVI and letter after candidate's name and be correct in 95% of cases. Boring!!!

2. Much less interesting primaries too. It's all but impossible in most cases to understand differences between lot of liberal Democratic candidates in democratic districts and lot of equally (usually - even more) conservative Republican candidates - in republican ones. Again - Boring!!

3. A lot of candidates that, obviously, don't fit their districts, like very liberal Democrats in rural South ( i don't speak about Black-majority areas here, though in the South they may have some conservative slant too) or conservative Republicans in New England. You can find very few such "odd" candidates in 60th - 70th, but very substantial number - now. That makes electrions even less interesting..

And so on. In general - if polarization will increase even more, the question "what do we need an elections for???" will arise... And it will be justified, because there will be absolute no substantial difference between candidates, who can win. All style and no substance...

[Mordecai]

Since the 1970s, the Republicans are moving to the right, both in economic and social issues. The Democrats are not moving to the left in economic issues, but they are moving to the left in social issues. So, American partisan politics is becoming more polarized from the 1970s to the 2010s.

However, the public opinion is not polarizing. The percentages of liberals, moderates and conservatives remained stable from the 1970s to the mid-2000s, and only after that the percentage of liberals and conservatives increased a little bit and the percentage of moderates decreased a little bit.

If the voters are not becoming very polarized, why did the parties become so polarized?

Gerrymandering.

[Fed. Pac. Chairman Devin]

All of the old southern Democrats have pretty much died off. That and most liberal Republicans have defected. Certain regions have run away from there party roots. Hell just 2-3 generations ago being a Republican in rural Mississippi would get you killed. Now you're just farmer joe on the corner. I wonder what caused the death of liberal Republicans?