In many Congressional districts, the primary is more important than the general election. In those districts, extreme partisan politics is rewarded and centrist politics is punished. This makes bipartisan legislation impossible, because a representative who votes for such legislation will be branded a traitor and voted out of office in the next primary.
But why is this so? In a simple model of politics (the real election, not the primary) voters are sprinkled left-to-right, and each of two candidates wins all the electors "closest to them"; which is to say, all the electors on the side away from the other candidate, and all the ones in-between that are closer to them. This model tends to push candidates towards the middle of the spectrum.
Now consider the primary. Exactly the same applies, except the electorate is different - most points are now either to left, or to right, of the center. So why do we see quotes such as the above (that do seem to have some basis in reality). I'm thinking of primaries in districts where one side is almost certain to win, so the primary is effectively the real election - but nonetheless, the same logic would appear to apply: you should get candidates pushed half-way out, not to extremes.