incumbent president - including Bush's son, George Walker Bush, who served from 2001 to 2009 - has been endorsed by the public when they have stood on their party's ticket.

It is a quirk of no small significance in a nation where eras are defined in the popular imagination by their presidents - from the thwarted promise of John F Kennedy's early 1960s to the cynicism and paranoia of the Nixon years and the thrusting optimism of Ronald Reagan's 1980s.

For voters and historians alike, the question of whether the head of state serves just four or the maximum eight years has huge symbolic value.

Donald Trump will be under pressure to run again and retain power come the 2020 election.

In a 2010 ranking of all 44 presidents by 238 eminentscholars for Siena College Research Institute, there were no single-term presidents in the top 10.

The highest-rated incumbent to have been defeated in a re-election campaign was John Adams in 17th place. Kennedy, in 11th place, was assassinated a year before he could return to the polls and James K Polk, in 12th, did not seek a second term.

Why do presidents serve two terms?

Image copyright Getty Images Image caption Bush served "four more years" under Ronald Reagan

Since World War Two, eight sitting US presidents have been re-elected to serve a second term, while only three have failed in a general election.

The presidency offers an unrivalled platform to attract airtime, raise campaign funds and set the policy agenda.

Sitting presidents, too, tend to escape bruising battles for their party's nomination -