The polite position in Washington policy circles is that politics has yielded to extremes in both parties. That is a really nice children's tale, but it is more than a bit absurd for anyone interested in reality. While the Republicans have adopted many extreme positions, Richard Nixon and Dwight Eisenhower would be very comfortable with the center position in the modern Democratic Party.
Michael Gerson helped to clarify the fundamental asymmetry between the two parties' positions in his column today. He complained that President Obama forced Congressional Republicans to vote for a tax increase for the first time in more than twenty years, violating their fundamental opposition to tax increases. (As a technical matter, the votes took place after taxes had already risen on January 1, so they were effectively voting for tax cuts for most households.) However, he complains that there was no quid pro quo with Democrats forced to support cuts to Social Security and Medicare.
This complaint shows well the nature of the asymmetry. While most Democrats are willing to support tax increases, especially on the wealthy, most Republicans consistently tell pollsters that they are opposed to cuts in Social Security and Medicare. In fact, there is not very much difference between Democratic and Republican attitudes towards these programs. The bases of both parties support them overwhelmingly.
Gerson is unhappy that President Obama refused to take a position that was not only unpopular with the Democratic base but also unpopular with the Republican base. This shows well the extremism of the Republican leadership in Washington.
It is also important to note that as a matter of arithmetic, support for Social Security and Medicare virtually necessitates higher taxes at some point, especially if health care costs are not brought under control. With a rising share of the population over age 65, these programs already cost considerably more now than they did forty years ago, measured as a share of GDP. They will cost still more in another twenty years.
Unless we are willing to see a substantial shrinkage in the rest of the government, it is impossible to support the increased spending on these programs without ever raising taxes. This makes the difference between Democrats and Republicans on this point primarily the fact that Democrats are willing to accept arithmetic whereas Republicans are trying to deny it.