Gordon Brown admitted he was the underdog for general election today, but insisted he was ready to "fight" for Britain.
The Prime Minister said voters faced a "big choice" between the Tories promise of "austerity" and Labour's plans to encourage "aspiration".
He also rejected claims that he was engaging in class warfare against David Cameron, and dismissed suggestions of splits in Labour's top team.
Asked whether he was the underdog in the coming fight, Mr Brown told BBC1's Andrew Marr Show: "I think when you are behind in the polls you have got to regard yourself as the fighter.
"Everything I have ever won in my life I have had to fight for...
"People will see they have a choice to make and the choice is between someone who is ready to fight for every aspect of Britain's future, and will not stop fighting."
Mr Brown said he "believed" there would be a Budget in the spring, but insisted he would not be "boxed in" on dates for a general election.
He also denied suggestions of tensions over campaign tactics between key Labour figures, such as Business Secretary Lord Mandelson and Commons Leader Harriet Harman.
"They are both going to be playing big roles in the election campaign, and every part of this Labour Cabinet is a team, it works well, and it is a team that will be fighting an election on the big issues," Mr Brown said.
Asked about persistent discontent among his party's rank and file, with former home secretary Charles Clarke having reportedly organised a "bring a rebel" event recently, Mr Brown said: "I don't really worry too much about that. The issue is not about internal debates in one party. It is about the country."
And he turned his fire on the Tories, insisting Mr Cameron had hoped to win without his own policies being scrutinised.
"I think the Conservatives wanted an election that was essentially a referendum on Labour. Then they thought they would get a referendum on the small issues.
"This is an election on the big issues."
The Prime Minister said the Tories had got "everything" wrong on the economic crisis, and would make the situation worse by cutting public spending too early.
"I believe in an age of aspiration, of opportunity and prosperity," he said. "I am afraid the Conservatives have gone for an age of austerity, and that means the majority of hard working families suffer as a result."
He added: "I've managed this country through good times and difficult economic times and we can manage this country through the next stage which is returning to growth, through higher levels of growth and higher levels of employment."
Quizzed about his barb at Mr Cameron in the Commons that Conservative policy on inheritance tax had been dreamed up "on the playing fields of Eton", Mr Brown played down its significance.
"It was a Commons joke. Quite an effective one, I hope," he said.
"I don't attack him for being a toff. I attack him for having the wrong views."
He added: "If you think the playing fields of Eton was anything other than a joke then I am afraid you take your politics too seriously."
With many predicting the election - due by June - could result in a hung parliament with the Lib Dems holding the balance of power, Mr Brown appeared to hold out an olive branch to the third party.
Quizzed on the possibility of a coalition, he said the Lib Dems were "closer" to Labour than the Tories on key policy areas.
"There is an agreement of ideas and of course the Liberals I think are closer to us on tax and public services," Mr Brown said.
"There is obviously the possibility of people working in common harmony. But equally we have got party politics there in the way."
He went on: "Our policies appeal to Liberal voters, because we are for Alternative Vote System, we are for reform of the House of Lords, we are for recall of Members of Parliament who commit fraud, and equally at the same time we have got policies on the environment and on civil liberties that are not dissimilar to them.
"But as for the fighting of the election, we, the Labour Party, will fight every inch of the way."
The intervention came after Mr Cameron made a similar veiled overture, stressing in his New Year message that Conservative differences with the Lib Dems had become less significant recently.