How much allure did such radical movements as communism and socialism have for americans in the 1930s? what was the "popular front" approach and why did it end?

Although communism and socialism were alluring for some Americans during the 1930s, especially with the failing economic system, these social ideologies never really took place in the US or never gained much public love due to the fact that the US has always been a heavily capitalistic country. 

The popular front effectively gathered men to fight the fascists in the Spanish Civil War. It ended eventually with the ending of the war.