Labour leader Jeremy Corbyn claims his party’s plans for a “green industrial revolution” will transform society and create high-quality jobs.
Mr Corbyn restated Labour’s support for schemes such as a Swansea tidal lagoon ahead of a visit to Machynlleth, Powys.
He accused the UK government of a “failed approach” to climate change.
And he said a no-deal Brexit would cause “massive” damage to the Welsh economy.
Prime Minister Boris Johnson has said the EU had become less willing to compromise on a new deal with the UK because of the opposition to leaving in Parliament.
Mr Corbyn said: “It’s going to be very, very difficult to see how the Welsh economy could sustain a no-deal Brexit.
“The issues of the interdependency of industry in Wales – Airbus, Ford, many other companies with Europe, as well as agricultural products – mean there always has to be a very close trading relationship with Europe.”
Mr Corbyn made the comments during a visit to the Centre for Alternative Technology in Machynlleth on Friday.
Before the visit, Mr Corbyn said Labour in power at Westminster would follow the Welsh Government’s lead on energy efficiency and in declaring a climate emergency.
Labour’s energy plans – outlined by Mr Corbyn at last year’s party conference – would also see the National Grid taken into public ownership and solar panels installed on nearly two million homes.
He said Labour policies would “benefit working-class people by cutting energy bills, creating good jobs in new, green industries and fighting the climate emergency”.
Mr Corbyn also criticised the Conservatives and Liberal Democrats for the scrapping in 2013 of a homes insulation programme for England by the then coalition government.
He claimed the cancellation of the Warm Front scheme for poorer households had cost them a total of £3.7bn in potential savings on their bills while increasing emissions by the equivalent of an extra 8.6m cars.
Mr Corbyn said that by contrast, the Labour-led Welsh Government had spent more than £240m since 2011 to improve household energy efficiency through its Warm Homes scheme.
Following the Welsh Government’s declaration of a climate emergency in April, the Labour leader led a Commons debate on a similar call in May.
The proposal was approved without a vote to demonstrate the will of the Commons to declare a climate emergency, but did not legally compel the UK government to take any action.
Jane Dodds, the newly-elected Liberal Democrat MP for Brecon and Radnorshire, said she backed Mr Corbyn’s plans.
“We absolutely would back any proposals, any initiatives which would make sure the Swansea tidal lagoon is back in place, with the funding it deserves, as well as any other initiatives,” she said.
“The climate emergency is one of those things we cannot afford to be tribal about. So anything that any party puts forward, is really to be supported.”