What would Einstein say if he were alive?
 ''I don't know, what?''
 Help, help!  Someone let me out of this box!

One of the FallaciousArgument''''''s, and a cousin of AdVerecundiam/ArgumentFromAuthority.  In the PosthumousAlly fallacy, a noted authority figure (usually one who is deceased, and thus in no position to disagree) is drafted as an ally.  This is typically done in the form "If X were still alive, I'm certain he'd agree with me", or else as a rhetorical question:  "If AlanTuring were still alive, what would he think about this problem?" or the ubiquitous "WhatWouldJesusDo?"

Sometimes, it is obvious that the PosthumousAlly would not likely be an ally as claimed.  The gunman recently executed in Florida (forget his name) for murdering an abortionist was quoted as saying that if Christ were present in the modern UnitedStates (where the courts do not enforce laws prohibiting abortion), he'd do the same thing (i.e. murder doctors who provide abortions).  Of course, Jesus was well-known for his pacifism and his "turn the other cheek" commandment--so it seems unlikely he would be popping caps into abortion doctors.

On occasion, the supposed ally is still alive (which leads to the risk that said "ally" might show up and issue a public rebuke to whoever claimed him/her as an ally).  I've seen Ward's name used in this fashion on this Wiki.  Personally, I would get annoyed if someone claimed me as their ally without consulting me first.  And I'm sure Ward would agree with me.
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Yes, just read an editorial claiming that were MartinLuther alive today, he would support GayRights and same sex marriage. -- BrucePennington