"Proposition" and "Proposal" are direct synonyms. However, in the example sentence, "Vote Yes on Proposition 136, but No on Propositions 129, 133, and 134.", they are not interchangeable. In the case of bills being introduced as a possible law, they are always called "propositions".
There are cases when one is preferred more than the other. You would seldom hear the term "marriage proposition", even though that would be a correct definition. It is nearly always called a "marriage proposal".