own weaknesses.

Carter was unfortunate enough to take office at a time when the global economy was in turmoil while Ford - who assumed office after Nixon's impeachment - only had two and a half years to make an impact, Sabato insists.

"Events put them in a bad position at the wrong time," he says.

It's a perspective that is flatly rejected by Robert W Merry, author of Where They Stand: The American Presidents in the Eyes of Voters and Historians.

In a democracy the customer - that is, the voter - is usually right, Merry argues.

He notes that with only a handful of exceptions - Grover Cleveland, John Quincy Adams - defeated presidents are rarely judged more favourably by historians than by the electorate which rejected them.

"The American people are veryunsentimental in their judgments," he says. "If you look back at one-term presidents, it's pretty hard to miss the reality that their performance was not quite there.

"Presidents get the blame and they get the credit. I'm uncomfortable with the idea that they are innocent bystanders."

As a result, he has little time for any suggestion that Bush was under-appreciated by the American public.

"I think he was a fine man, but he was an in-basket president," Merry says. "He responded to stimuli that came to him. He didn't have an agenda to change America in any particular way."

Certainly, Franklin D Roosevelt, Reagan and Obama all inherited economies in dire straits and all won re-election.

What is not in doubt is that, in an average year, an incumbent enjoys significant advantages over