David Cameron has described Chancellor Gordon Brown, describing him as an "analogue politician in a digital age", adding: "You are the past".
The Tory leader was replying to the chancellor's Budget speech, claiming Mr Brown had "taxed too much, borrowed too much" and been "a roadblock to reform".
"We wondered whether we'd get a budget or a leadership bid," said Mr Cameron. "We didn't get much of either."
He said his party welcomed some of the plans, which he claimed had Tory roots.
Mr Cameron branded the chancellor's spending forecasts as "a mess".
Personal savings were down and the government was borrowing £175bn over the next six years, he said.
"This chancellor is mortgaging this country's future - that's over £6,000 of debt for every household in this country," he said.
"You are an old-fashioned 'tax and spend' chancellor and that approach is completely out of date.
Mr Cameron criticised Mr Brown for giving the UK the "biggest tax burden in history".
"The story of this week is of a prime minister and a chancellor up to their necks in debt, making promises you can't keep and not knowing whether to work together or fight with each other as the ship starts going down," he said.
The "real challenge" was competing in the global economy, but he said Mr Brown was not the right person for the job.
'Billions raised, billions spent'
The UK used to have the "10th lowest" business taxes in the world, but now has the "10th highest", said Mr Cameron.
Despite having 10 budgets to boost science, there were science departments in universities closing.
"You have had 10 budgets to improve transport and some of our motorways look like car parks.
"You cannot be the change this country needs because you are the architect of the policies that have put us where we are.
"Billions raised, billions spent. No idea where the money has gone.
"With a record like that you should be running for treasurer of the Labour Party," he said, referring to the row over secret loans to the Labour Party.
The Tory leader told Mr Brown it had not been a genuinely green budget, "and that's not surprising because you haven't made a single major speech on the environment in 11 years".
"In a carbon-conscious world, we have a fossil-fuelled chancellor," he said.
He mocked Mr Brown's pledges of help for pensioners, adding: "The chancellor who smashed up the pension system can't be the one to rebuild it."
Mr Cameron added: "You may see yourself as the rock upon which Labour can rebuild their church - instead you are the roadblock stopping Britain from meeting the challenges of the future."
At just under 10 minutes, Mr Cameron's speech was the shortest Budget response by a Conservative leader in recent political history, according to the BBC's political research unit.
His predecessors Michael Howard, Iain Duncan Smith and William Hague came in at 16, 19 and 22 minutes respectively.