Why do nearly half of Americans vote Republican?

[haele]

23. You mean "Why do half of voting Americans vote Republican?"

There's several reasons -
1. A large number of voters who might vote Democratic - or Green, or Libertarian, or any party other than Republican for that matter - are currently disenfranchised by disingenuous "voter verification requirements" passed by partisan legislatures wishing to consolidate power.

2. Gerrymandering ( i.e., depending on the last re-districting based on Census 2010, where pretty much a majority of Republicans came out to vote), which currently benefits more Republican districts than Democratic. Voters in gerrymandered districts are either wildly over-represented - and their votes don't count for any candidate outside of their district or are totally overwhelmed - so their votes also don't count within the district.

3. Group-think prodding using fears or resentment to motivate like-minded participants to get out and vote. Churches, community "clubs" and like social organizations will often create a competitive "us against them" philosophical reality to goad their membership into action to ensure their organizational goals become the primary goals of the community at large, no matter what would actually serve the population of that community best. So long as they have their army of fellow travelers and willing servants/warriors, they think they can continue to increase the amount of respect and power they gain over everyone else. It's a typical conservative/authoritarian world-view - so long as everyone keeps to their "place", and any opposition is ruthlessly crushed before it can grow, everything will be just hunky-dory.

4. The comfort of Status Quo. It's easier to motivate action against a clear "threat" than motivate action to "do something positive", even if it is obvious that without doing something, society at large is in the process of hurtling towards a metaphorical cliff unless steering, breaks and windshield wipers are added to the vehicle.

As to what makes a voter decide to vote Republican now-a-days?
Family Tradition. Organizational Belonging. Fear of Social Change or loss of "prestige" (position in the social hierarchy). Single Issue voting.
Any of these can be behind why people vote Republican.

Haele