Political scientist: People become unenthusiastic about voting if they believe that important problems can be addressed only by large numbers of people drastically changing their attitudes and that such attitudinal changes generally do not result from government action.
Conclusion (thus)
The decreasing voter turnout is entirely due to a growing conviction that politicians cannot solve the most important problems.
Evidence
If people believe that important problems can then people be addressed only by → become large numbers of ppl unenthusiastic drastically changing about voting their attitudes and that Big attitudinal changes usually don't result from government action.
Evaluate
Well, this feels like a mess. When we're doing Flaw and we see conditional logic, we want to immediately consider the #1 famous Necessary vs. Sufficient flaw. Does the author use her conditional premise in an illegal backwards or negated fashion?
Does she seem to be thinking, ?
No. So she's not doing an illegal negation. How about an illegal reversal? Is she arguing, Maybe in a diffuse way, but not in any obvious way.
The author's conclusion discreetly establishes that there has been decreasing voter turnout recently. This would be a Curious Fact that would prompt an author to try to Solve the Causal Mystery, and the author's conclusion is doing just that. It's saying,
Curious Fact ... what's causing decreasing voter turnout?
Author's Explanation ... it's exclusively being caused by people's belief that big problems require big attitude shifts combined with people's understanding of the fact that big attitude shifts rarely come from government action.
Goal
We can object to / weaken this argument by pointing out some Alternate Explanation (even if it's in addition to the one the author supplied) for decreasing voter turnout.
Or we can object to / weaken this argument by attacking the plausibility of the author's causal story that people believe big problems requires big attitude shifts and don't believe that politicians or government action can trigger those attitude shifts.