Liberal Democrat leader Tim Farron has called for a major programme of infrastructure investment as he accused the Conservatives of planning cuts that go far beyond what is needed to tackle the deficit.
Mr Farron said deficit reduction measures taken under the previous coalition government meant the country could now afford an "active, ambitious and targeted" programme of capital spending.
In a speech to the Institute for Public Policy Research think-tank, he said Chancellor George Osborne should take advantage of current low rates of interest to invest in transport, housing, education and high-speed broadband.
The Lib Dem leader said Tory plans for £100 billion in cuts over the next five years - to be set out in next week's spending review - were almost double what was necessary to tackle the structural deficit in the public finances.
He accused Mr Osborne of being driven by "dogma and short-term politics" and called for a "reprioritising" of the Government's approach to deficit reduction.
"The cheap borrowing costs we see today are a direct result of the prudent deficit reduction we undertook in government, so that past austerity makes future austerity less necessary," he said.
"Borrowing now to invest for the future can make debt more sustainable, not less. Or, to put it another way, fix the roof while the sun is shining.
"If you've got 0% interest rates then the sun is shining. Why don't we fix the roof?"
Mr Farron, who is seeking to rebuild support after calamitous general election losses, sought to position the Lib Dems as the party of entrepreneurs with a series of measures to encourage investment and help small business.
He said Conservative claims to be the party of business were a "complete and utter myth" while the "liberal gut instinct" was to be on the side of the entrepreneur.
"The fact is that the Tories aren't really pro-free market capitalism at all. They are pro-corporate capitalism," he said.
"They are there to fight not for entrepreneurs, not for innovators who oil the wheels of the market, but for the status quo.
"The Liberal Democrats will be the party of small business, the party of wealth creators, the insurgents, the entrepreneurs."