Labour leader Ed Miliband will condemn David Cameron as "petty and small-minded" and talk about creating "a new kind of Labour party" in his keynote speech at the TUC conference.
Miliband's speech is likely to be among the most important of his career, as he seeks to recover from a series of blows to his credibility over recent weeks.
He will try to redefine Labour's relationship with the unions, give more definition to his 'One Nation' concept and attack Cameron's style of politics.
A press release from Labour ahead of the speech said that Miliband will link the reform of trade unions with the re-energising of a broader left-wing movement of people at the grassroots.
Labour and the Unions
Members of the Labour frontbench and some senior union leaders have clashed over what sort of political influence unions should have over the creation of party policy.
In an effort to head off his critics, Miliband will say "we are going to have to build a new kind of Labour Party" based on "a new relationship with trade union members."
Delegates will be told that "the vast majority [of members] play no role in our party. They are affiliated in name only. That wasn't the vision of the founders of our party. I don't think it's your vision either. And it's certainly not my vision.
"That's why I want to make each and every affiliated trade union member a real part of their local party, making a real choice to be a part of our party so they can have a real voice in it."
Whether union members will buy what Miliband is selling is open to question after a summer where he was criticised for keeping his profile far too low while his opponents scored political points.
Another major objective of Miliband's speech will be to strengthen the 'One Nation' brand that he introduced to distinguish himself from New Labour.
A part of this strategy is to attack David Cameron as a leader removed from the everyday worries of voters, while he is more in touch with the traditional working class base of Labour.
"It is you who have been telling me year after year about a politics that is detached from the lives of working people. We need to build a party truly rooted in the lives of all working people in Britain once more."
The One Nation Conservatives spearheaded by Benjamin Disraeli in the Victorian era "would be turning in their graves if they could hear the nasty, divisive, small-minded rhetoric of the leader of their once great party.
"We have a Prime Minister, who writes you and your members off. Who doesn't just write you off, but oozes contempt for you from every pore. What does he say about you? He says your members are a 'threat to our economy'. Back to the enemy within.
"Six and a half million people in Britain. Who teach our children, who look after the sick, who care for the elderly, who build our homes, who keep our shops open morning, noon and night. They're not the enemy within. They're the people who make Britain what it is.
"How dare he? How dare he insult people, members of trade unions as he does? How dare he write off whole sections of our society?
"We know from recent experience what happens to political leaders who write off whole sections of a country. That's what Mitt Romney did when he talked about the 47% of people who would never vote for him. And look what happened to him. Friends, my job is to make sure that's what happens to David Cameron as well."
In what might be either a clever ploy of political clothes-stealing or a weak public relations gimmick, Miliband appears to be trying to make himself more electable in three ways.
One is to recast 'One Nation' Labour as more friendly towards unions and the concerns of everyday voters than New Labour, which is seen by some as having moved the party's ideology too far to the right.
A second is to refashion the Labour and trade union relationship so that he cannot be attacked by the Tories as being a mouthpiece for the unions or insensitive to the concerns of union members themselves.
The third and most subtle aspect of Miliband's 'One Nation' brand is to associate the potential patriotic feeling around the 'One Nation' concept with the left-wing side of politics and not the right.
Miliband might be thinking of an earlier Tory leader other than Disraeli who was very good at fusing emotions of British nationalism with her style of politics.
In the 1980s, Margaret Thatcher seemed to be electorally successful in tapping into a core of conservative and patriotically-minded working-class voters. Many of them seemed to be readers of The Sun.
Perhaps Miliband is attempting to achieve a similar coup as he articulates his vision of 'One Nation' Labour.
Whatever happens, Miliband has a lot at stake in his speech to the TUC audience.