Three sources of radicalization in the Congressional Republican caucus: replacement of Southern Democrats, replacement of Republicans by more conservative Republicans, tendency for Republican members of Congress to grow more conservative the longer they are in office.
Party leaders have actively sought to promote policies that satisfy the conservative base, and they have organized the led and relied on the base to weed out or bring into line Republican politicians who are too moderate.
Increasing political influence of the wealthy. Turnout becoming more heavily tilted to the most well off. Money is becoming a much more important factor in political races as campaigns become more expensive, and this, of course, is an even bigger boost to the influence of the wealthy. Trade unions and civic groups once provided an organizational counterweight for the less well-off, but they have largely decayed.
Congressional and Senatorial seats have increasingly become safe for one party or the other -- most often, Republicans. Some of this is due to historical trends in party affiliation (like the movement of the South to Republicans). But on the district level, a great deal of it is the result of partisan redistricting. This means that Republican incumbents generally have more to fear from primary challenges than general elections opponents.
Activist groups representing base constituencies within the GOP coalition increasingly drive turnout. They also are playing a larger role in recruiting and vetting candidates.
Political parties are actually becoming a more important factor in candidate success, largely because they have become a centralized source of money. Money from Republican Party leaders' PACs are an especially important factor in driving primary and general election success of right-wing candidates.
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