a challenger in a US election, thanks to the visibility and prestige of their office.

But that raises the question of what difference a second term actually makes, in practice, to a president's legacy.

Image copyright The White House Image caption One-term father, two-term son

Re-elected presidents are, of course, freed from the requirement to face the voters again, which may offer them some leeway. The very fact they have been endorsed twice at the polls can enhance their authority.

But they face the same institutional barriers and separation of powers as in their first four years - the mid-term elections, the need to win Congress's support for legislation.

"The president has some more leeway to make decisions in a second term, but they begin to lose their starch," says Zelizer.

"You find fewerpeople who are willing to take a job in their administration for a two-year tenure. It's harder to maintain forward momentum."

As a result, he says, the defining policies of most presidents tend to occur in their first term.

In Sabato's view, George HW Bush should not be regarded as a one-term president but as the president who spearheaded a three-term dynasty.

"He had a son who served two terms and served them only eight years after his own," he says.

"I don't think there's any question they viewed it as a personal vindication of the 1992 defeat."

And given the unpopularity of George W Bush when he left office, it would appear that many people may believe Bush Senior's one term made him a better president than his two-term son.