I, and many others, have noted the swing to extremism, particularly in the Republican party since the advent of Newt Gingrich. The current bevy of GOP presidential candidates demonstrates this in spades. Here's a new, potential, explanation:
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Although the House and Senate are quite different from electoral and institutional standpoints, they share one important thing in common: the same members. In the current Senate, about half of senators first served in the House. Examples include staunch liberals such as Barbara Boxer of California and Richard Durbin of Illinois as well as staunch conservatives like James Inhofe of Oklahoma and David Vitter of Louisiana. Notably, this seems to be a modern development. In the 80th Congress (1947-1949), just 19 percent of senators came from the House.
How the House has made the Senate more polarized.

I don't think it is inconsequential that the of the candidates are Senators with some pretty extreme views. It also makes me wonder if Governorships follow the same pattern of radicalization. It's sort of self-reinforcing.

I think the pattern is much less pronounced on the D side, as even Sanders, although self-identified as aa "Social Democrat" is still, all in all, pretty mainstream.

A well reasoned argument is like a diamond: impervious to corruption and crystal clear - and infinitely rarer.

Here, as elsewhere, people are outraged at what feels like a rigged game -- an economy that won't respond, a democracy that won't listen, and a financial sector that holds all the cards. - Robert Reich