The classic example would be the 1968 election in which the Democratic Party split over Vietnam. A push by delegates at the Convention in Chicago, lead by Eugene McCarthy, to change the official policy to withdraw from Vietnam lead to violence on the floor of the Convention itself. The division ultimately contributed to the election of Republican Richard Nixon.

Meanwhile, outside the Convention, more than 10000 anti-war protesters were attempting to make their views known. A large number of police were present and tears gas was deployed and protesters beaten.

A bit of primary evidence won't hurt here - this is some video of the riots. You can get a sense of the scale and the police response.

You might like the book Chicago '68 for a good introduction if this interests you. The view of the police can be obtained from Battleground Chicago: The Police and the 1968 Democratic National Convention

The whole year was hard - assassinations of MLK and Robert Kennedy. Anti-war protests, riots in Paris. Eight protest leaders ('the Chicago 8') were later put on trial and jailed.

Yes. You might even say that violence during American presidential campaigns and elections was endemic until the 20th century.

In the period between the end of the Civil War and the 1920s, reactionary and conservative elements in the American South carried out a sustained and successful campaign of political terrorism designed to disenfranchise blacks and lower-to-middle-class whites. The Klu Klux Klan is known today, but it was one of many groups designed to roll back the progressive voting rights laws implemented after the end of the American Civil War through violence, intimidation and fraud.

In this case, terrorism absolutely worked. You can see it in the voter rolls. Turnout in the South fell from 64 percent in the presidential election of 1876 to 50 percent by 1900. It dropped to 32 percent by 1916, then just 20 percent by 1924. Poor whites and all blacks were effectively blocked from voting, creating the "Solid South," a Democratic bloc of states that lasted until 1964 on the presidential election map.

Foner's books on Reconstruction are some of the best for learning about how voter intimidation worked during this period and geographical area of American history.

But election violence was a constant presence even before the American Civil War. There was pro-slavery violence against abolitionists, there was nativist violence against pro-immigrant groups (and vice versa), there was violence between parties.

With regard to the nativist violence, turn to the presidential election year of 1844. In that year, the annexation of Texas and the extension of slavery was the top issue, but boiling under the surface was the issue of nativism. The United States was in the middle of a huge surge of immigration that was changing the demographics of the country. The new immigrants were mostly Catholic; the established ones mostly not.

In Philadelphia, things boiled over with some of the bloodiest civil insurrection until Kansas in the 1850s. In July, armed rioters and militia clashed repeatedly, and many were killed or injured. As Zachary Schrag, a historian studying the riots, writes:

"Although American cities, particularly Philadelphia, had endured a surge of riots since the early 1830s, few individual riots lasted for more than a day, making the 1844 riots extreme in their severity and duration. While some of the violence had been spontaneous, the ambitions of the nativist newspapers and political party in an election year likely sustained nativist fury through the spring and summer. Though the riots were more than the simple transplantation of anti-Catholic violence from Northern Ireland, they echoed the deliberate provocation seen there."

The Philadelphia riots were a precursor to the electoral violence surrounding the nativist parties like the American Party and the Know-Nothings in the late 1840s and early 1850s. As former British cabinet minister James Buckingham wrote in 1841: "one of the strongest of the national prejudices of the mass of the people of America, embracing all classes except the highest and most intelligent, is a dislike ... of all foreigners."

When looking at historical sources and comparing them with modern events, take caution not to fall into bias. We have a great deal of information about the present and recent past, but our ability to access information from the more distant past is limited. Individual assaults and violent actions were not covered in detail as they are today. We do not have video record of such events, and we do today.

As a follow-up, was there ever a point in time when protestors at a presidential campaign rally would be "carried out on a stretcher" as certain candidates have claimed?