A Partial History of Democratic Party Presidential Succession
This is a useless bit of Democratic Party historical trivia, I suppose, but that's the type of stuff that tends to grab my interest. No Democrat has succeeded another Democrat into the presidency in their own right since James Buchanan succeeded Franklin Pierce in 1857.
While it's true that Harry Truman succeeded Franklin D. Roosevelt, that's only because Truman was Roosevelt's Vice President when Roosevelt passed away. The same is true of Lyndon B. Johnson when he succeeded John F. Kennedy after his death. Grover Cleveland couldn't even succeed himself, being the only person in U.S. history to serve two non-consecutive terms as President of the United States.
In the case of Pierce and Buchanan, there might also be an asterisks, however, as Pierce was running for reelection when he lost the nomination of the Democratic Party to Buchanan. So it's not as if Pierce had served his full two-terms, not that two-terms was a rule at the time, and then Buchanan was elected when Pierce was done. No, Buchanan won the Democratic nomination from an incumbent Pierce and forced him into an early retirement.
If we don't count Buchanan succeeding Pierce, then we have to go back to 1837, 20 years earlier, when Martin Van Buren succeeded Andrew Jackson after Jackson's full two-terms. You might then be able to argue that this is the only time a Democrat succeeded another Democrat, because prior to this there was no Democratic Party, per se. There was the Democratic-Republican Party of Thomas Jefferson, more commonly referred to at the time as simply "Republicans," but no Democratic Party. That said, the Democratic-Republican Party was essentially just rebranded as the Democratic Party in Jackson's time so I would not personally make that argument.
It seemed as if Hillary Clinton might break this streak in 2016 as she ran to succeed Democratic president Barack Obama, but that was not to be as Republican nominee Donald Trump ended up defeating her to become the 45th President of the United States. The earliest this streak could hope to be ended is probably 2028, and that assumes that Joe Biden will defeat President Trump in 2020 to become the 46th President of the United States and then win reelection in 2024. No small tasks.
Regardless, it's an interesting bit of presidential trivia that Democrats have not been able to win the presidency for more than a single president's time in office, absent a death in office, for 163 years.