Political scientists and pundits generally expect centrists to hold sway. This has not happened because the institutional moderating forces have not been as effective as expected.
1) voters -- not only are voters poorly informed, but this ignorance is not random. Republicans strive to make sure voter ignorance is tilted in their favor, which has diluted the effectiveness of voters without strong ideological affiliations as a moderating influence.
2) opposition -- the Democrats are simply not as unified and organized as the Republicans. Senior committee members more autonomous. Campaign funding often depends on special interests with an agenda that cuts against liberal policy. Moderates vulnerable to Republican framing. Lack of institutional control means there is little ability to reward loyalists, while Republicans can reward those who stray. Disproportionate significance of Republican-leaning small states in Senate apportionment creates a large segment of vulnerable Democratic Senators who are especially hard to keep in line.
3) media -- decreasingly effective in exposing the actual effects of Republican policies because of a) focus on entertainment over substance, b) "he-said she-said" model of objectivity, and c) herd mentality in deciding what issues are worth covering. The parallel right-wing media universe also plays an important part in distorting coverage.
4) Republican moderates -- intimidated and bought off.
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