Nationally televised debates of US presidential candidates are usually boring stalemates - but not always. Former US president John F. Kennedy famously looked cooler and more presidential than his rival Richard Nixon in their 1960 debates, which proved crucial in one of the closest results in US election history. Another former US president Jimmy Carter lost big to Ronald Reagan in 1980 and former US vice-president Al Gore's supercilious sighs clearly tilted support to former US president George W. Bush in 2000 when the latter needed it most.

US President Barack Obama's tired, lackluster performance in this year's first presidential debate with former Massachusetts governor Mitt Romney may prove an equally important turning point in history.

Over the previous month, Romney appeared determined to throw away a campaign he had previously had an excellent chance of winning. His speech at the Republican National Convention in Tampa, Florida, didn't give him boost or bounce at all in the opinion polls.