The President who was elected to the office in years divisible by 20, would die while in office. Where it was an actual curse or not, since 1840, the elected President has died during his term, whether of accident, disease, or assassination. Kennedy in 1960, FDR in 1940, Warren G. Harding in 1920, William McKinley in 1900, James Garfield in 1880, Abraham Lincoln in 1860 and William Henry Harrison in 1840.
But something stayed the weight of the curse in Reagan's case. In the normal course of things, he should have died when shot by Hinckley. Whether it was by the bullet being deflected just enough by the body armor Reagan was wearing or a touch of Providence, Reagan survived.
Yep - and by beating the curse, he has made the presidency safe for GWB.