Franklin D Roosevelt in 1932 became the first presidential candidate to accept in person the nomination of his party at a convention. He set the bar high for everyone else who followed with his brilliant speech that promised 'a New Deal for the American people'. Nominees ever since have tried to replicate FDR's level of rhetorical impact. Many thought that 2008 would be another landmark year for convention oratory. Few, however, predicted that the star performer would turn out to be the governor of Alaska.

Political junkies have long been immersed in the current race for the White House. The convention season is traditionally when everyone else tunes in. And boy did they tune in this time round. A TV audience of 37 million saw Barack Obama's address in Denver. That Sarah Palin, only a prospective VP, got the same viewing figures was completely unexpected. With numbers like that, it is no wonder these speeches are high-wire acts that unnerve even the best speakers – the usually flawless Obama fluffed a number of lines, including "I accept your nomination for the presidency of the United States."

Of the four presidential and vice-presidential speeches, the stakes were clearly the highest for Palin (and by extension the man who had picked her). She had no experience of speaking at a prime-time nationwide event or to an audience of such a size. The media had been roughing her up on her personal life and lack of national experience. Her speech was a one-off opportunity to stake her claim to be a heartbeat away from occupying the Oval Office.

Her speech was quite simply brilliant. At turns folksy, self-deprecating, and ballsy, it encapsulated the new strategy that the Republicans will hammer between now and election day: we are maverick outsiders who will shake up the Washington old boys' club. In a 'change' election, that is a powerful message. And to add style to substance, Palin was mischievous and funny. Her line about the difference between a hockey mom and a pitbull – "lipstick" – combined all the wit, feistiness and homeliness that she wanted to portray in a simple one-liner. All that and an image that commentators are calling the 'sexy librarian' look – no wonder the Democrats are worried.

Palin's superb speech made McCain's life easy on acceptance night. A brilliant speaker at 'town hall meetings', he has never been comfortable delivering set-piece speeches to large audiences. He hates the autocue and struggles to get his lines in sync with audience reactions. But because Palin had provided the star quality, his understated performance reinforced his maverick image. I am a war hero and an outsider, his speech said, if you want political slickness go elsewhere. It was a tactic that worked for Eisenhower in 1952, running against media darling Adlai Stevenson.

That Barack Obama appreciated the simplicity of the McCain brand was evident in his own convention speech the previous week, even before Palin had been announced on the Republican ticket. His demeanour that night suggested regret at the rally style of the stadium event. The Republicans had scored a palpable hit with criticism of his 'world tour' and 'rock star' status. Obama smiled very little during his address and used almost no humour. His message too was clear enough: I am ready to be commander in chief. It was, said Ronald Reagan's speech writer, Peggy Noonan, "a muted affair, but not one without power." In retrospect, he may regret ceding to Palin the tag of plucky outsider that had worked so brilliantly for him in the primary campaign.

Obama's running mate, Joe Biden, who has working class roots, missed a trick there too. Does anyone even remember a speech that was overshadowed on the night by President Bill Clinton? It was a solid enough affair, although less polished than we have come to expect from this veteran senator. At the time it seemed to matter little. What counted was not what Biden said, but who he was: voters concerned about Obama's lack of experience in foreign affairs could be reassured by the presence on the ticket of the chairman of the Senate foreign relations committee. That reinforced Obama's own message: ready to command. Between the two of them, they managed to define themselves as the candidates of 'experience'. That was before the Republicans stole their clothes of 'change'.

The convention season belongs to Sarah Palin. She electrified the Republican party in a way unseen since Ronald Reagan announced himself at the 1964 convention. Yet what she harnessed was not so much Reagan's glamour, but something closer to the appeal to 'folks' of small town America that Harry Truman distilled so effectively in the 1940s.

In January 1945, Truman was sworn in as Roosevelt's vice-president. Less than three months later, FDR was dead and Truman was commander-in-chief. "Watch the president," US fleet admiral Ernest King reassured Winston Churchill shortly afterwards. "This is all new to him but he can take it. He is a more typical American than Roosevelt, and he will do a good job."

After Palin's accomplished speech, Republicans think they've found their own Harry Truman. Or as the hockey mom herself might say: "she shoots, she scores."

Richard Aldous is the author of 'Great Irish Speeches'