Poverty and underdevelopment are the cause for Marxism or is it politics that fan them?
You can definitely argue that poverty and underdevelopment cause Marxism or at least cause people to believe in Marxism. Marxism offers A) a group of people to blame if you are poor and B) hope that you, as a poor person, will get your "fair share" of money and goods if a Marxist state comes to power. This analysis argues that only poor countries will ever have large Marxist movements. You could say that the fact that there's little Marxism in US politics comes from our prosperity.
However, you can also say that Marxist politicians/activists keep it alive. France has a significant Marxist presence and France is not really poor. In that case, you'd have to say that they continue to be Marxist because politicians convince them that it's the best way.
Marxism refers to the thinking and ideas of Karl Max, These are predominantly concerned with the nature of economic systems and ways of managing and governing economies and countries. Any spread or contraction in extent of application of Marxism in the government methods and policies, by definition, has to be a political action. One may ascribe spread of Marxism to politicians fanning it. But that would be as meaningless as saying that politicians fan democracy, or politicians fan desire for freedom.
Poverty is definitely a condition that makes the promise of Marxism very attractive to people. But It should be noted that Marxism is more concerned with disparities in wealth and income between different classes of people in the society, rather than the absolute levels of poverty. Marxism is more concerned with equitable distribution of income between all the people in a society rather than with increasing the total combined income of the society as a whole.