This paper estimates the cost effectiveness of climate activism as being in line with the cost effectiveness of clean air task force. https://forum.effectivealtruism.org/posts/fttiTrTnrjCuqFNvw/fighting-climate-change-with-progressive-activism-in-the-us
My reasoning for donating is not based on those assumptions (1) money can increase the chance candidate X is elected and (2) candidate X’s being elected will produce a lot of good.
I'm not trying to replace the people in power. It is more effective to lobby the people already in power by taking full advantage of the existing system.
I have donated to specific candidates in the past and I actually regret that. It is much more effective to donate to a PAC like peace action. They can be much more strategic in spending and most importantly candidates can see how much they get from peace action and know where their bread is buttered. If I donate personally, my name doesn't tell them what the money is for.
Also, let's think in concrete terms about what these campaigns pay for. They pay for ads which can raise awareness. Some campaigns like the Bernie, Yang, and Ron Paul have impacted the conversation without winning.
But it is even more effective to donate to 501(c) 3 tax deductible activist organizations like code pink. They raise awareness of issues at a grass roots level. They lobby and they take advantage of volunteers. Plus it's tax deductible and you can get employer matching.