This idea is really interesting. Thanks for posting it and it would be cool to see if it worked!
One reason people might be hesitant to do this is people might think that donation-canceling succeeds in not changing the odds of victory for either candidate, but might believe that politicians pay more attention to the beliefs of their donors. If that were true, elected politicians would pay less attention to those with EA-sympathies. Even if it's not true (political scientists are pretty skeptical of that claim), enough people might think it's true to be an issue.