It’s looking good for Hillary Clinton’s presidential bid. Polls show an increasing national lead, and her campaign is pushing into states that wouldn’t usually be considered competitive. There have even respected figures effectively calling the election for Clinton already, weeks from polling day.

Prevented by a 1951 constitutional amendment from running for a third term himself, Barack Obama has campaigned hard for a Clinton victory. Clinton is not running for Obama’s third term and any victory would be her own, not his. Indeed, it is Michelle, not Barack, Obama who been called Clinton’s “most effective surrogate” in campaigning terms, and her appearances have been so successful there have been suggestions, and even assumptions, that she will one day run for national office.

Yet everyone is aware that Obama’s achievements in office, particularly Obamacare, are more easily secured by his replacement coming from his own party, indeed someone who served in his administration at a senior level, and the Obamas have not been reluctant to use their popularity to try and help achieve that outcome.

The energy the Obamas have put into Clinton’s election is understandable. If historical precedents mean anything, then the Obamas are right to be worried. Should Hillary Clinton win, Barack Obama would become the first Democratic President to be succeeded by a member of his own party without dying in the process for more than a century and a half.

This is not just a matter of the pendulum nature of US politics, ie. that the retirement of a sitting President means more people consider switching parties. The Republicans have generally been better at securing the succession than the Democrats.

There is a related situation on this side of the Atlantic; no Labour Prime Minister who attained the office mid-parliament has ever yet gone on to win the subsequent general election; this is not something that Conservative Prime Ministers appointed without a national election have had the same trouble with. Nor are they likely to in the immediate future.

In 1989, George HW Bush, in many ways the epitome of the Republican establishment, moved smoothly from being Ronald Reagan’s Vice President to the presidency, while 60 years earlier, Herbert Hoover, Secretary of Commerce under Presidents Warren G Harding and Calvin Coolidge succeeded the latter in 1929, ensuring the White House remained in Republican hands.

Twenty years before that, Theodore Roosevelt had successfully campaigned for William Taft, his chosen successor, to win the presidency. Not that that ended well either. The men later fell out and Roosevelt ran in 1912 as a third party candidate, destroying Taft’s attempts to remain in office and ensuring the election of Democrat Woodrow Wilson.

Yet Taft is better remembered for being the only President who was also later Chief Justice of the Supreme Court, one of only two Presidents buried at Arlington National Cemetery, and so awe-inspiringly fat the White House needed to replace its bathtubs with larger models during his single term of office.

In living memory, the presidency has only passed between Democrats through the death of the incumbent POTUS. Kennedy’s assassination in 1963 raised his Vice President, Lyndon Johnson, who won a landslide in his own right a year later. In 1945, four times elected Franklin Delano Roosevelt (yes, he was from the other party to the other President Roosevelt) died of a massive stroke three months into his fourth term.

Neither Woodrow Wilson (1913-21) nor Grover Cleveland (1885-89, 1893-1897) were able to secure the succession, despite each being elected twice (Cleveland’s two terms being interrupted by the single-term presidency of Republican Benjamin Harrison).

Andrew Johnson (1865-1869) came to the presidency at Abraham Lincoln’s assassination, the two having run on a multi-party national unity ticket while the civil war raged. Impeached by the senate, Johnson was unable to secure his own renomination and reelection.

In 1857, James Buchanan, the last Democratic President before the civil war, succeeded fellow Democrat Franklin Pierce. But the unpopular Pierce was refused the nomination by their party, who looked to give it to Buchanan, a party man who had conveniently been US ambassador to London during much of Pierce’s administration.

It’s a measure of how decisive a break with Pierce’s government Buchanan made that he replaced the entirety of Pierce’s cabinet, despite being of the same party and despite them being then, and now, the only cabinet to serve a full presidential term without a single resignation or replacement.

This means the last Democrat POTUS to see out his term of office and hand over to a successor of whom he approved and for whom he campaigned was Andrew Jackson. He retired after two terms at an election that saw his long-time campaign strategist and later Vice President Martin Van Buren elected as his successor.

This is sufficiently long ago that Jackson was the last President who could remember the revolutionary war and Van Buren was born during it. The latter succeeded the former, regarded by history as the first President from the Democratic Party, on 4 March 1847.

That’s so long ago, it’s roughly the last time the pound sterling was worth what it is now.

Too much can be made of electoral precedents like this. Until Harding was elected in 1920 it was thought that no sitting senator could be elected to the presidency, although only two have subsequently. And it was an article of faith among southern Democrats that Sam Rayburn, the long-serving Speaker of the House of Representatives, would have been president were he not handicapped by being a southerner, it being assumed that after Reconstruction no southerner could be elected president. Rayburn died in 1961, and there have been multiple southern presidents since, beginning with his protégé, Lyndon Johnson.

There are many other examples of these sort of “never haves”. This XKCD comic strip, which came out during the 2012 election, demonstrates exactly how far the idea can be taken. On that basis, while ending the Democrats’ 140 years of successional failure isn’t the best or most important reason to elect Hillary Clinton President, it would be nice to be able to tick another one off the list.