The Welsh MP insists that another decade of Tory rule will leave Britain in a bleak state.
Mr Smith was saying: “As we move in to the final weeks of this campaign, I wanted to spell out exactly the risks facing our country if Labour continues with Jeremy Corbyn as leader, failing in its duty to offer a powerful opposition to the Tories and not looking like a credible government in waiting.
“This would offer the Tories a certain victory in 2020, with Labour becoming an unelectable political party. It will make the prospect of a Labour government in 2025 even more unlikely and unleashing a further wave of brutal right wing policies.
“Today I’m setting out in detail what an unencumbered Tory Party would look impose on the country. This would include a massive expansion of grammar schools, further extending the use of the private sector in the NHS, the effective end of social housing and the use of the Brexit negotiations to dismantle hard won workers’ rights.
“We can only avert this by voting for change at this election. Yes we must maintain a radical anti-austerity agenda, but back it up with concrete policies to deliver change that can unite our party and the country. That is what I am offering in this contest.”
The remarks came as Mr Corbyn insisted Britain needs a £300 million advance research agency to keep it at the cutting edge of high-technology.
Mr Corbyn wants the US-style project to make breakthroughs in green technology and combating climate change.
The Labour leader said the initiative would be aimed at attracting some of the globe’s finest minds to do their research in the UK.
Mr Corbyn said: “It is crucial for both energy security and tackling climate change that we give the most serious investment and incentives to high-tech R&D.
“My pledge to establish a dedicated advanced research agency will help tackle the global social challenge of climate change and make fundamental breakthroughs in energy science.
“It will put the UK in the best possible place to take advantage of the rapidly growing global renewable energy market, forecast to reach 630bn US dollars worldwide by 2030.
“The Tory cuts to the UK renewables sector - including dramatically cutting feed-in tariffs and subsidies for domestic installation - are nonsensical and a damaging backwards step in our efforts to decarbonise our energy production.
“My pledge is about ensuring that the UK is at the forefront of world-leading technology development in a sector of growing demand.”