The wounded President could have survived today. He may have survived then had new equipment, on display at the Pan-American Expo, been available for him.
The biggest medical problem was the bullet that had penetrated his stomach. Doctors couldn't find it. Complicating the search was the President's size. He was a big man.
Trying to locate and remove the .32 caliber projectile, the attending surgeon, Dr. Matthew Mann (a gynecologist who had never before operated on a gunshot victim), probed in vain for the bullet.
As doctors tried to save the President, anxious crowds waited outside the Exposition's Emergency Hospital.
An X-ray machine was exhibited near the spot of the assassination. If only doctors had known how to use that machine, it likely would have saved McKinley's life. (Follow this link to an early 1898 machine. Note where the technician is seated. It was before the time when pioneering scientists were aware of the dangerous side of X-rays.)
Discovered by Wilhelm Conrad Roentgen at the University of Wurzburg's Physical Institute Laboratory on November 8, 1895, X-rays have saved countless lives since then. (We can still see the famous December 22, 1895 radiograph of Mrs. Roentgen's hand. It was the X-ray Dr. Roentgen submitted as proof of his discovery.)
In an altruistic gesture that may seem strange today, Roentgen (who won the Nobel Prize for Physics the year McKinley was shot) refused to take out a patent on his invention. He wanted the whole world to benefit from it.
Unfortunately, the whole world did not include President McKinley. As gangrene set into the abdominal wound, his prognosis worsened.