In the United States, threatening the President is considered a class D felony. It appears to have its origins in the British Treason Act, making it a crime to "compass or imagine" the death of the King.
The scope of what falls under threat and what the punishment should be is hotly contested. Some feel it is within their first amendment rights to wish death upon the President and others believe only physical assaults should fall under the Class D crime label. Still others believe the existence of such pages as Facebook group "LETS KILL BUSH WITH SHOES" and polls asking if President Obama should be assassinated or not are an affront and a threat that should not be allowed.
Numerous threats have fallen upon our various Commanders-in-Chief. From the World War I and World War II eras, in which nationalism was high and the fear of spies very prevalent, the government was very attuned to and strict with laws relating to threats on the President. Some convictions based on verbal threat or call for death upon the President include a poster advocating that passerby hang FDR. Before said poster, the declarations "President Wilson ought to be killed. It is a wonder some one has not
done it already. If I had an opportunity, I would do it myself"
and "Wilson is a wooden-headed son of a bitch. I
wish Wilson was in hell, and if I had the power I would put him there" led to convictions.
Some of the more controversial calls for threats on the president include comedian Groucho Mark's assertion that "the only hope this country has is Nixon’s assassination." And when President Clinton vetoed a ban on partial-birth abortions, a pastor told him "God will hold you to account, Mr. President." The Secret Service detained the man, reportedly declaring the incident a threat on the President's life.
Other threats that provoked outrage from the public and attention from the Secret Service include cartoonist Michael Ramirez' depiction of a man with a gun to President W. Bush's head in the LA Times. Though the Secret Service gave him a visit, he was not arrested. In contrast, a University student, Vikram Buddhi, was arrested for his Yahoo messages calling for "the assassination of GW Bush" and to "Rape and Kill Laura Bush." After serving 11 months, we was promptly re-arrested for having an invalid green card.
Finally, in a more recent incident, a man was arrested for his Craigslist post stating "People, the time has come for revolution. It is time for Obama to die. I
am dedicating my life to the death of Obama and every employee of the
federal government. As I promised in a previous post, if the health care
reform bill passed I would become a terrorist. Today I become a
terrorist." The post brought the man under fire for its obvious references to the potential endangerment of the President and US citizens.
However the scope of presidential threats and the punishments doled out will be treated by the government, there will continue to be threats on the presidents' lives, just as there are on any leader in any country. No one will ever fully agree on how to handle such cases, and surely there will be some threats with no real danger behind them as well as others that should be immediately addressed. But due to the United States' strong Secret Service and security system, our Presidents are much safer than in times past.