'Jumping the Shark' was first defined when a long-running popular TV show does something hugely stupid and after that moment the show pretty much sucked. It goes back to Happy Days, once a very popular staple of American TV viewing, when they had the Fonz jump a shark tank. There are websites devoted to 'Jump the Shark' moments in TV including when shows add a new baby to the show, when too many popular characters leave a show or when a popular show adds Ted McGinley to their cast; there are many many reasons as to why really good shows start to suck.
'Jump the Shark' has been used for decades even beyond TV shows to justify how something was once very popular and then turned to crap because of a stupid choice made.
To imply that Mitt Romney has 'Jumped the Shark' means that at one time Mitt Romney was actually a decent, viable candidate that was well liked by Americans. I don't think he was ever well liked even by the people who voted for him. I think for many Mitt Romney was simply 'Anyone But Obama'. The absolutely racist type hatred towards Obama has made these right-wing nut jobs settle for Romney because after that long, drawn-out primary season all their favorite candidates pretty much took turns shooting themselves in the foot and dropping from the race.
So to me Mitt has not 'Jumped the Shark' because he's not even close to the Fish Tank where the shark swims and his motorcycle is busted to pieces.
Nuff said.