What happens if a presidential candidate is killed before an election?

(NEXSTAR) — On Saturday evening, former President Donald Trump was rushed off the stage of a campaign rally after apparent gunshots rang out and he appeared to be injured. While the Trump campaign said Trump is "fine," the incident sent major shockwaves through the political world.

"An incident occurred the evening of July 13 at a Trump rally in Pennsylvania, the U.S. Secret Service said in a statement on Saturday. "The Secret Service has implemented protective measures and the former President is safe. This is now an active Secret Service investigation and further information will be released when available."

Unfortunately, the possibility of a presidential candidate being assassinated or harmed isn't new to campaigns. In 1912, former President Teddy Roosevelt was shot during a campaign speech in Milwaukee — though the bullet lodged in Roosevelt's chest, the former president immediately removed a bloody 50-page speech he had tucked where the bullet entered, which he credited with his survival. He finished his speech with a bullet inside him.

A much more serious incident happened on June 6, 1968, when Democratic presidential candidate Robert F. Kennedy was shot several times in Los Angeles. Kennedy's gunman was captured, and Kennedy was pronounced dead the next day.

As far as presidents go, every modern president has survived potential assassination attempts and/or planned incidents, including Trump. In 2017, the Secret Service thwarted a reported plan by an Islamic State of Iraq and the Levant (ISIL) affiliate to attack Trump during the 2017 ASEAN Summits in the Philippines.

While there aren't rules governing what happens if and when a presidential candidate is assassinated, there is general guidance about what happens if a candidate dies. The Brookings Institution explains that it depends on when a candidate dies or is unable to participate in their elections.

If primaries have not been held yet, a particular party might need to push back the events to allow a new pool of candidates to step into the arena. If a convention has already selected a candidate who ends up being unable to run, that particular party would then need to select someone new — and both the Republican National Committee and the Democratic National Committee have different ways of going forward.

On Saturday, Butler County District Attorney Richard Goldfinger said one rally attendee was killed and the suspected gunman was also dead, according to AP.