What makes a great political speech?

All eyes will be on Ed Miliband today and much has been written about the importance of his party conference speech.

But what makes a good political speech? Inevitably, Ed Miliband will be compared with Labour leaders of the past, particularly Tony Blair who was known for his persuasive powers. Phil Collins, who wrote many of Blair's speeches, says that great political speeches need a big event or a rallying cause and there are just less of them than there were in the past.

He will be speaking in a debate on political rhetoric at this year's Cambridge Festival of Ideas next month. Other speakers include David Runciman, reader in political thought at the University of Cambridge, author Piers Brendon, former Keeper of the Churchill Archives Centre and Michael White, the Guardian's political editor. The event will be held at Churchill College, Cambridge on October 20th.

For Collins, great political speeches need three key ingredients: a serious argument which leaves the audience thinking something new or resolved to act; great delivery that stirs the emotions as well as appealing to reason; and a sense of occasion.

He says: “Martin Luther King could get away with elevated language because his cause was a noble one. You can’t really do that when you are talking about the reform of local government. It just isn’t as big an affront to justice. So, there is a very good reason we have fewer remarkable speeches which is that we don’t need them as much as we did.”

Collins also justifies the use of sound bites, although he says he always worked by building a solid argument first and then trying to distil the best possible phrase out of the argument rather than the other way around. He says that not only are soundbites vital in a world where a 24/7 media edits chunks of speeches down to one phrase, but all the great writers are full of them. “We should guard against the derogatory association of the word soundbite,” he says. “All we mean, really, is a pithy way of capturing the essence of the point. To be or not to be – that really was the question. It was a soundbite too.”

He adds that the emphasis on soundbites is likely to increase. “The endless fragmentation that results from the coverage of modern media is the main reason that the soundbite has become such a ubiquitous part of political discourse. Your words are going to be chopped into pieces in any case so you might as well offer up the encapsulation you think is the best one.”

Collins says that one of the potential pitfalls of modern party conference speech is the number of people who vet it. “The big conference speeches have many authors, or at least many contributors,” he says.  “It is inevitable, when there are lots of hands at work, that the integrity of the argument goes missing. The task for a conference speech is always to recuperate the argument. The more a single person can be in overall control, as a sort of editor-in-chief, the better. Writing by committee is rarely a good way to work.”

Nevertheless, a good political speech can make all the difference. David Cameron owes his leadership of the Conservatives to two speeches, he says – one he gave which was well received and one given by his rival David Davis which “bombed”. He adds that it is hard to imagine Barack Obama would have become President without his oratory powers.

The audience is clearly vital for any speech writer and Collins says people's attention spans have declined, as has the breadth of their vocabulary and range of reference. Mass democracy means that references to  high culture divide an audience where they would once have united it, he says. There are also more political speeches than there used to be.

“Gladstone and Disraeli used to speak rarely every year. Each speech was an epic, months in the preparation, but they would not be doing speeches three times a week, as many politicians are now,” he says. “In the process, we have devalued the currency a little. The effective political speech, though, remains what it has always been – a mixture of reasoned argument and emotional passion.”

Other speakers at the Festival of Ideas debate will focus on the historical or wider issues associated with political speech-making. Piers Brendon, for instance, will talk about Churchill's use of political rhetoric, which he likens to the style of a music-hall performer, and contrast it with today's more colloquial, television-orientated and soundbiteish delivery.

• The event, to be held at Wolfson Theatre, Churchill College from 6-7.15pm on Thursday, 20 October, will be chaired by Allen Packwood, Keeper of the Churchill Archives Centre. Arrive at 5.30pm to see an exhibition of documents from the Centre.