With modern medical care, James Garfield absolutely would have survived his wounds. In fact, a case could be made that Garfield would have been better off if nobody even tried to treat him after he was shot. It was unsterilized instruments and dirty fingers being poked into Garfield’s wounds in an effort to find the track of the bullet which introduced the infections that eventually killed him. Of the four Presidents who were assassinated, Garfield’s original wounds were the least severe, and the gunshot wound that Reagan survived in 1981 was less severe than both Garfield’s and McKinley’s. Charles Guiteau shot James Garfield, but it was the President’s doctors who killed him.
Lincoln is a different story. Sure, there’s always the possibility that with an immediate response and modern technology, Lincoln could have remained alive via life support, but he would have been in a vegetative state. But even that is highly unlikely. John Wilkes Booth shot Lincoln at almost point-blank range. There were powder burns around the wound in the back of Lincoln’s skull, so Booth was very, very close — probably less than six inches away from the President — when he shot him. Also, it’s important to remember that Lincoln was shot with a Civil War-era weapon, so he wasn’t struck with a bullet as we think of bullets today, but a .41 caliber ball from a Derringer pistol. The ball flattened as it passed through the back of Lincoln’s skull and carried bone fragments from the skull as it passed through the full length of Lincoln’s brain. There was no exit wound and the pressure and trauma fractured both of Lincoln’s eye sockets (from inside of his head). The ball probably lodged in bone near or behind Lincoln’s right eye — several attending physicians immediately after the shooting and in the autopsy the next day disagreed about which eye the bullet lodged behind because as they opened up Lincoln’s skull during the autopsy, the ball literally fell out of his head and through the fingers of a doctor. Most believe that it lodged behind the right eye because it was protruding after the shooting.
Lincoln’s wound was fatal in almost every instance in 1865, and it would probably be just as fatal in 2014. He kept breathing until 7:22 AM the next morning, but Lincoln was also known to be a surprisingly strong man physically so his respiratory system put up quite a fight. However, it’s pretty much a given that Lincoln was brain-dead by midnight or 1 AM; the trauma to his brain was too severe — the autopsy showed that the bullet not only passed through the entire brain and carried skull fragments with it, but it also left sharp pieces of bone from his skull in various parts of the brain. The doctors attending to Lincoln acted immediately and did as good of a job as could be expected — in 1865 or 150 years later — but it was a fatal wound.
Reagan was very lucky for the quick reaction of the Secret Service. At the Washington Hilton, the lead agent, Jerry Parr, quickly shoved the President into the limousine as shots rang out and Tim McCarthy did exactly what the Secret Service is supposedly trained to do — he instantaneously turned towards the gunman, made himself a bigger target, and literally took a bullet for the President. If Parr hadn’t shoved Reagan into the limo when he did, Reagan likely would have taken a bullet to the head; instead, two bullets hit the limousine and one of them ricocheted off the side and struck Reagan. If it wasn’t for the quick reaction of the Secret Service during the shooting and immediately afterward when Parr diverted the limo to the hospital instead of the White House, Reagan would have died from either a more direct shot or from the massive blood loss that he was suffering from. The doctors at George Washington Hospital believed Reagan would have died if he had arrived at the hospital even just a couple of minutes later. It also helped that Reagan was in really good physical shape for a 70-year-old man. But if Reagan had been shot in 1881 instead of 1981; he’d have ended up with the second-shortest Presidency in history. And if Garfield had been shot in 1981 instead of 1881, he wouldn’t be the guy holding that record.