Were any of the presidents covertly murdered?

[Senator Incitatus]

It seems interesting that in the scope of 250 years, the United States has never had a president killed by any method other than a dramatic public execution by firearm.

Please state your theory in the comments. If you'd like to offer an option not listed, explain how he was replaced by a body double.

[Skill and Chance]

Harrison and FDR died of what were known to be untreatable illnesses in their day, so this is profoundly unlikely.  An FDR conspiracy is intriguing because that was such a momentous time in the world, but he was known to be sick and it was well past the point in the war where an Allied victory was ensured.  It was also too late for a Soviet-led conspiracy because Wallace was no longer VP.  FDR dying suddenly in April of 1943 (Axis) or 1944 (Soviets) would have been a lot more suspicious.

Using modern forensic techniques on samples from his remains, it has basically been ruled out that Taylor was poisoned.

Harding is more interesting, because he would be expected to survive the heart attack with 1920's medical treatment had he been in D.C. or a Western city when he got sick.  It appears his wife pressured him to continue with the remote Alaska trip despite knowing he had symptoms of heart disease flaring up.  She was dealing with the fallout from his string of affairs so they were not on good terms by this point.  Also, she intervened to block an autopsy. 

[Senator Incitatus]

Harrison and FDR died of what were known to be untreatable illnesses in their day, so this is profoundly unlikely...

Some, including the doctor who actually made it, have questioned Harrison's pneumonia diagnosis.

https://www.nytimes.com/2014/04/01/science/what-really-killed-william-henry-harrison.html

[Sir Mohamed]

Nah, there's little reason to believe so. There were and still are some rumors around Warren Harding, though, that he was murdered by his wife because he cheated and her and after she refused an autopsy.

Taylor for sure his disproven after the 1990s exhumation.

[Orser67]

Seems unlikely to me that any of them were assassinated, though I'd agree with a couple other posters that Harding's death seems like the one where foul play was most likely involved. Ironically, in terms actual impact on the country, Harding's death may have had the least impact of the four as Coolidge generally continued his policies. Harrison's death essentially ended a Whig trifecta, Taylor's death led to the passage of the Compromise of 1850, and FDR living into the late 1940s is, if nothing else, an interesting alt-history scenario.

[NewYorkExpress]

I could easily see Harding's wife getting convicted of manslaughter if someone had actually bothered to bring charges against her (blocking an autopsy on his corpse doesn't seem suspicious at all), but the other three seem like normal deaths.

Also, William Henry Harrison is an idiot. Even I would know not to give an almost two hour long inaugural speech outdoors in freezing March weather and wander around in a rainstorm, again in freezing cold weather.

[○∙◄☻¥tπ[╪AV┼cVê└]

Harding maybe.
Garfield was murdered by his doctors' incompetence.

[Joe Republic]

Also, William Henry Harrison is an idiot. Even I would know not to give an almost two hour long inaugural speech outdoors in freezing March weather and wander around in a rainstorm, again in freezing cold weather.

It was normal for military men to be used to being outside in appalling weather for long periods of time.  What Harrison (and Washington, who went a very similar way) didn't realize that his advanced age didn't allow for the same level of resilience as in his younger days.

[Skill and Chance]

Also, William Henry Harrison is an idiot. Even I would know not to give an almost two hour long inaugural speech outdoors in freezing March weather and wander around in a rainstorm, again in freezing cold weather.

It was normal for military men to be used to being outside in appalling weather for long periods of time.  What Harrison (and Washington, who went a very similar way) didn't realize that his advanced age didn't allow for the same level of resilience as in his younger days.

Yes, there were no truly waterproof raincoats or tarps until the Civil War era, so this was just a fact of life at the time.  It would likely have been considered feminine to complain about it.

[All Along The Watchtower]

no