It probably depends on what you consider an
assassin or an attempt. Some people are politically motivated, while others are mentally ill and might use politics as an excuse. Some get close and get a shot off and are clear attempts, while others are stopped far away or are questioned after expressing a desire without ever doing much.
* Lincoln was a Republican.
* James Garfield was a Republican, shot by someone who was mentally ill.
* William McKinley was a Republican.
* John F. Kennedy was the only Democrat assassinated.
Attempts against Republicans* William Taft's had his would-be assassin caught feet away and disarmed.
* Teddy Roosevelt, a Republican when he wasn't in his own Progressive Party, was shot, but his pocket contents slowed the bullet and it was lodged between his ribs but didn't kill him.
* Herbert Hoover almost had his train bombed in Argentina.
* Richard Nixon was a target by man who couldn't get through security, but who later successfully shot and maimed Governor Wallace of Alabama. Another would be assassin wanted to crash a hijacked plane into the White House, but was stuck on the ground where he killed a couple pilots.
* Gerald Ford had two attempts within 17 days. One was a Charles Manson follower, but didn't know to chamber a round in her gun, the other's aim was ruined when someone in the crowd grabbed their arm to stop them.
* Ronald Reagan, as we all remember, was shot and sounded by a mentally ill person, but survived.
* George H. W. Bush was the target of an Iraqi car bomb plot, which was foiled.
* George W. Bush had a hand grenade thrown at him in Tbilisi, Georgia and some dude shot at the white house.
Attempts against Democrats* Franklin D. Roosevelt was shot at in Chicago, but they were never sure if he was the target or if it was a mob hit against Chicago Mayor Anton Cermak. There were alleged wartime plots.
* Harry Truman was attacked by two Puerto Rican Activists at the Blair House, while the White House was being renovated. He also received letter bombs from Zionists, as did other world leaders.
* Jimmy Carter was the target of a mentally ill person with a starter pistol and blank rounds. Not sure that counts.
* Bill Clinton was the target of several plots. One guy plotted to shoot him while jogging, another crashed a small plane into the White House, another shot at the White House, and Osama Bin Laden plotted to blow up a bridge but they rerouted after intercepting a message.
* Barack Obama,
had multiple threats, with several people jumping the fence and a plot by some Army privates who were serious enough to kill 2 people who were a threat to their OpSec.
It looks like Republicans are targets more often. This makes sense to me. Leftists are often loner types of radicals. Leftists tend to live in cities, where they are alone in the crowd and depend on impersonal systems that are more often targeted by GOP and capitalists.
Rightists tend to be communitarian, which means they are caught beforehand or talked down by friends and family and never make the news or eye of law enforcement. Rightists are often rural and that insulates them. It gives them a siege mentality, but that's a defensive stand in case the gubmit comes for them, and gives them a sense of security that can dissuade them from acting.
I think a lot of the hollerin the Right did was explicitly to try to create a hysteria against Obama because analysis of his policies shows he wasn't a threat to most of what they claim to hold dear. Thus, few rightists had grievances, real or imagined, worth the effort. Leftists, by contrast, often have more to lose from their perspective, which makes them more likely to act. For people who believe in Global Warming, the fate of the world and the future of humanity is literally at stake.
With Trump, it could be either side, a military coup, or foreign actors. If Trump is smart, he'll ram his stuff through, claim victory, and resign, allowing Pence to ascend to the Targetcy.