This is just my opinion...

[muriel_volestrangler]

58. Poll: Partisan Polarization Surges in Bush, Obama Years

As Americans head to the polls this November, their values and basic beliefs are more polarized along partisan lines than at any point in the past 25 years. Unlike in 1987, when this series of surveys began, the values gap between Republicans and Democrats is now greater than gender, age, race or class divides.

Overall, there has been much more stability than change across the 48 political values measures that the Pew Research Center has tracked since 1987. But the average partisan gap has nearly doubled over this 25-year period – from 10 percentage points in 1987 to 18 percentage points in the new study.

Nearly all of the increases have occurred during the presidencies of George W. Bush and Barack Obama. During this period, both parties’ bases have often been critical of their parties for not standing up for their traditional positions. Currently, 71% of Republicans and 58% of Democrats say their parties have not done a good job in this regard.
...
In recent years, both parties have become smaller and more ideologically homogeneous. Republicans are dominated by self-described conservatives, while a smaller but growing number of Democrats call themselves liberals. Among Republicans, conservatives continue to outnumber moderates by about two-to-one. And there are now as many liberal Democrats as moderate Democrats.

http://www.people-press.org/2012/06/04/partisan-polarization-surges-in-bush-obama-years/

So it does seem that there is a demand for more separation; but this also seems to be a new thing. Consider that there used to be Eisenhower and Rockefeller Republicans. That 71% of Republicans saying their party doesn't stand for its 'traditional' positions are ignoring that kind of Republican, and concentrating on people like Barry Goldwater, or fringe loons like the John Birch Society, and deciding that is their 'tradition'.

This page goes into more detail about the divide: http://www.people-press.org/2012/06/04/section-1-understanding-the-partisan-divide-over-american-values/ . Democratic shifts have been to become more supportive of immigrants, more concerned about equal opportunities, and less religious; Republicans care far less about a social safety net, and the environment, and hate unions far more than they used to.

It seems to me, however, that most candidates reflect these pretty clearly. If there were still moderate Republicans around, you might have a point - Democratic candidates might look similar to them. But moderate Republicans are like coelacanths - a few of them hidden away, and largely seen as 'living fossils'.