If I remember correctly, there were two attempts on President Ford's life, one of which was the infamous Squeaky Fromme - one of the Manson Family. Left wing nut.
There was the guy who shot at the White House during Clinton's term, but I don't remember his specific political affiliation. I think he was up there with Hinkley, who was just a nut for nuttiness sake.
Sirhan Sirhan was actually a Arab socialist-nationalist, and a big supporter of Palestinian nationalism. Like Nassar and Saddam - both of whom were socialists - he would be considered a left-wing nut.
James Earl Ray is now running around claiming that he didn't kill MLK. And I think I saw a recent show that mentioned that MLK's family is also receptive to the notion that Ray didn't kill King. So, this leaves this assassin in doubt.
Arthur Bremer attempted to assassinate Nixon, and did eventually shoot Wallace. He was a nominal left-winger who cited Wallace's pro-segregation stance as a reason for killing him.
Skinheads wanting to off His Excellency Barack (Idi) Hussein (Amin) Obama will be a relatively new phenomena.