I have completed an analysis of officials from the major American political parties (Republicans and Democrats), scoring them against the criteria in my policy platform for total welfare.
The conclusion is that actions which increase the political power of the Democratic Party (like voting for them) have a substantial positive expected impact. The conclusion is insensitive to plausible differences in opinion about the relative weightings of political issues and conventional global priorities. I also consider and reject arguments for voting for minor candidates.
I also briefly address this debate with some alternative methodologies: deontological ethics, deference to public opinion, deference to expert opinion, and deference to EA community opinion. All of these alternative methodologies suggest that efforts to increase the political power of the Democratic Party are justified.
This doesn't address the relative importance and tractability of boosting Democratic power compared to other EA cause areas, but I think it does meet a reasonably high standard of evidentiary robustness, comparable to that of numerous other EA cause areas.