Thanks for this! 

First came across a similar argument in Joe Carlsmith's series on expected utility (this section in particular). The rest of that essay has some other interesting arguments, though the one you've highlighted here is the part I've generally found myself remembering and coming back to most. 

I think I feel a bit confused with the concept of it being crucial to  "stack a bunch of multipliers." 

Like how would you describe this story: 

A student is planning a career working on US economic policy (because this is something he's had an interest in for a little while). Then, he is exposed to some ideas in longtermism and decides to instead work on AI policy instead (because he thinks it seems like a big deal and is probably more important than economic policy).  

 This feels to me like it's just "one step" or "one multiplier" that puts this student on a much higher EV career path. 

Curious about this part:

“To be concrete, my personal best guess is that >30% of American EAs would have at least a decent shot (say, >20%) of making it into Congress if they chose to dedicate themselves to their local community and spend time practicing the appropriate communication skills.”

Would be curious for a bit more explanation from OP or others on this part.