A base canard.
Damn near verbatim quote from my High School history teacher! 
Not quite an already-elected candidate, but in 20th-century Mexico, being the PRI (party) nominee amounted to the same thing: in 1993, Luis Colosio was assassinated a few months before he surely would have become president (even if, as in 1988, this would have required a little, ahem, creativity with the Election Day computers).
Sadly, ruined by it sounding like a medicine.
Lady Jane Grey was deposed nine days after becoming England’s de-facto queen, and executed about seven months later. Not exactly an assassination, but the end results were the same.
Peter III of Russia ruled for exactly six months, when he was taken into the woods (apparently on his wife’s orders) and had a “hunting accident.” His wife, Catherine II (a.k.a. Catherine the Great) went on to rule for 34 years.
Gabriel Narutowicz was sworn as the first President of Poland on December 11th, 1922, and officially took over from Józef Pilsduski (Acting Head of State) on the 14th, and was then assassinated on the 16th.
Well, no, Richard had deposed Edward two and a half months into his reign. He killed him later, after Edward was no longer king.
Keeping score …
These two seem to be the winners for the OP’s first contest. The future has its work cut out to dethrone Narutowicz from the dubious honor of his first place finish. The event in 1922 has the further advantage that it’s recent enough that we have credibly accurate records to that level of precision.
Depending on who you believe, the man who should have been Carlos IV of Navarre.
Carlos, first Prince of Viana, was named second in line for the succession of Navarre by his grandfather, Carlos III of Navarre. First was his second daughter Blanca, whose oldest sister had predeceased Carlos III. The title of Prince of Viana was granted to him by Carlos III to signal him as his heir; it is now one of the automatic titles of the heir to the Spanish crown.
When Carlos died, Blanca became indeed Queen. Her husband, who later became Juan I of Aragon, had problems understanding that did not make him King of Navarre: in Navarre, his job was that of consort, and like any other consort he was expected to shut the hell up and look pretty unless asked. Juan hated the Prince, thinking him weak and unmanly (the Prince took after CIII, both of them being what today we’d call geeks, more fond of mechanical things and books than of gutting their enemies).
When Blanca died, her will indicated that the throne was to go to the Prince, but that he could not be crowned until Juan gave permission. Juan, being some words I shouldn’t put in writing, refused the permission on grounds of “I am the king!” There were some arguments along the lines of “no damnit, here you’re not”… 20 years after Blanca’s death and while preparing to meet with his father in Barcelona, the prince took ill and died. The unofficial story is that he was poisoned; the official story is that he died of tuberculosis caused and aggravated by the conditions of his captivity under Daddy Dearest. In either case, Juan is definitely not a front runner for “Father of the Year”, at least if you ask his firstborn (Ferdinand of Aragon may have a different opinion).
Well, wasn’t he “just” Regent, or Lord Protector, or something? What about the kids in the butt of malmsey? One of them was King, no?
Hazy History supplied by Hazy Memory of the Bard.
Richard was Lord Protector for his underage nephew. However, two and half months into Edward V’s reign, the Bishop of Bath and Wells came forward to announce that Edward’s father had been previously contracted to another lady, making his marriage to young Edward’s mother invalid and the boy king illegitimate, and thus, not king. The nobles and the citizens of London petitioned Richard to become king, an act affirmed by Parliament the following year.
Young Edward and his brother remained in the old royal residence at the Tower of London, and eventually disappeared from sight. Their fate remains unknown.
The butt of malmsey was the alleged cause of death of a different member of the family, George, Duke of Clarence, who was a brother to Richard and to Edward IV (father of the boy king). George was arrested, convicted, and “privately executed” at the Tower for treason in 1478, by order of Edward IV, who personally prosecuted his brother.
He was a duck?
Thanks. Wow, hazy memory indeed.