It has to be illegal because people are voting for representatives, not draft picks to be designated at a later date. What that is doing is delegating the selection of a representative to one individual, not the constituency at large, even if they are complicit in the choosing. And, in fact, they are not complicit in the choosing because those who voted for an opposing candidate did not vote to have a blank voted in, they voted for a live, breathing body. Even if their candidate loses, they went to the polls expecting to vote for a known candidate who campaigned, also expecting other voters to go to the polls and do the same. So, even if one side plays wink-wink-nod-nod, they are corrupting the process for everyone who goes to the polls.
-PJ