Final answer:
McCarthyism was primarily fueled by the government's apparent support for McCarthy's claims against Communists. This support came from prominent Republicans who regarded McCarthy as an asset to target Democratic politicians.
Step-by-step explanation:
Senator Joseph McCarthy is considered the main reason behind McCarthyism due to various factors. One key point is that the government appeared supportive of McCarthy's claims. Prominent Republicans like Senator Robert Taft and Congressman Richard Nixon regarded McCarthy as an asset and supported his actions. McCarthy's accusations against government agencies were seen as an effort to target Democratic politicians. The government's support further legitimized McCarthy's campaign and allowed it to gain momentum.