The writers of the Constitution tried to accomplish this by limiting the power of the central government. They would be appalled if they could see what their vision has become.
Was that a plug for your "the Constitution wasn't written by the finger of God" beliefs?
The problem is, as Greg pointed out, the primary systems being dominated by the most politically active among the constituents guarantees the nomination of candidates who pander to the extremists. Not those who want a functioning, bipartisan government. In the past, it was commonplace for candidates to feign support for the official platforms only to immediately drift centerwards once elected. Now that's the recipe for defeat in the next primary at the hands of someone who swears they won't betray the platform.
*MySmiley*
"Bustin' makes me feel good!"
Ghostbusters, by Ray Parker Jr.