Unluckiest U.S. politicians

[Lincoln Republican]

Lincoln's first Vice President Hannibal Hamlin.

Generally, an incumbent President will keep his Vice President on the ticket when he seeks reelection.

However, Lincoln was somewhat dissatisfied with Hamlin, and due to the prevailing circumstances, Lincoln felt it more beneficial for the nation to pick the southern Democrat Andrew Johnson instead, so he dropped his Vice President, Hamlin, who but for Lincoln's decision would have become the 17th President of the United States. 

[Lincoln Republican]

Lincoln's first Vice President Hannibal Hamlin.

Generally, an incumbent President will keep his Vice President on the ticket when he seeks reelection.

Actually from Calhoun to Marshall no Vice President won the second term.

Richard M. Johnson, Hamlin, Schuyler Colfax and Levi Morton were all dropped from the ticket by an incumbent.

Well, I did say generally. 

[Lincoln Republican]

Robert Todd Lincoln, son of Abraham Lincoln and Secretary of War under James Garfield.

Robert Todd was 21 when his father was assassinated.

1881, only four months after becoming Secretary of War in the Garfield cabinet, Garfield invited Lincoln to go with him for a Presidential visit to New Jersey.  Before boarding the train, Garfield was shot,  with Lincoln there, and Garfield died shortly afterwards.

1901, in Buffalo, New York, Lincoln was invited to a meeting  by President William McKinley, where McKinley was shot and died a few days later.  Lincoln  was in the room at the time of the shooting.

Lincoln turned down all future Presidential invitations.