Jumping the shark is a metaphor that was originally used to denote the tipping point at which a TV series is deemed by a viewer to have passed its peak. This happens when the creative energies of writers have been exhausted or when the inherent stupidities of the plot have been fully realized.
We can use that expression even outside popular culture if anything got past its peak or has got worse or made a grave mistake. For example, Conservapedia jumped the shark when they let TK back in autumn 2008 and since then even Conservatives who were silly enough to be loyal to CP have left on droves and are now at A Storehouse of Knowledge.
In the political sense it means that someone or some organization has said or done something so incredibly stupid as to be irrelevant in the wider scheme of all things political, such as George W. Bush getting elected president of the USA and the invasion of Iraq. We'll have to wait and find out what Donald Trump will do.