What a hypocritical, nasty creep, he was (1841-1845).
Tyler was called the 'accidental president' because he came in as William H. Harrison's VP, and Harrison died a month into office. As Harrison's illness (probably pneumonia, which he came down with at his inauguration) hung on for a month, and his death was determined very likely, if not inevitable, Tyler had a goodly span of time to strategize how to take the presidency for himself. Tyler was ANOTHER Virginian ....
At that time no POTUS had died while in office. There were no procedures in place as to what to do. The question had come up among the power players at times: hold another election was favored at least as much as any other procedure. What Tyler did then, became precedent for all other presidents who died while in office. If I were in the Senate or the House right now, I'd be studying what Tyler did very carefully.
Tyler's regime was a disaster, one of the long string of presidential disasters since the 1830's, as slavery became an ever more distasteful and every growing blockade to union and solutions to all sorts of other problems the nation faced.
What, if anything, people remember about Tyler, along with being the first president to take office because the elected president died, is manifest destiny and expansionism, the lead-up to the Mexican American War, which war was conducted under the administration of Tyler's successor, James K. Polk.
Tyler's justification for expanionism went like this: Slavery is a problem, but the problem is that there are so many slaves in our southern states we don't know how to deal with them. If we have more! new! territories into which to expand! slavery! then the population of slaves in the older slaveholding states like Viriginia! will be thinned as the slaves are taken away to open the new territories. This is a good and grand solution to everything.
The farkin' hypocritical, lying, sleazy bastard from Virginia didn't mention that slaveholders like himself made so much income from selling slaves that expansion meant expanding that market for their slavebreeding industry.
He was a vocal early proponent for secession. When he died in 1862 he was a seated member of the confederate congress. So much for his sincerity to preserve the union and end slavery.
The recent (2006) John Tyler: The Accidental President, by Edward P. Crapol (University of North Carolina Press) provides a well-written, readable discussion and analysis of his character and presidency.