Labour has been accused of making "wild, outlandish claims" on the threat to jobs in Wales if the UK leaves the European Union after a referendum.
Lord Hain, unveiled as manager as Welsh Labour's campaign to stay in the EU, said 200,000 jobs could be at risk.
Prime Minister David Cameron is pushing for EU reforms ahead of a referendum he has promised by the end of 2017.
Welsh Secretary Stephen Crabb said the Tories were trying to bring some "cool rationality" to the argument.
Launching Welsh Labour's EU campaign in the Vale of Glamorgan on Monday, Lord Hain said: "It's not just the risk to high-quality jobs in Wales from employers who treasure our place in the EU, like Airbus and Toyota in the north and Ford and Sony in the south, that a vote to leave the EU would bring.
"At present Wales benefits by over £838m every single year. That's money invested in our communities, supporting vital infrastructure and supporting Welsh businesses and farmers."
First Minister Carwyn Jones added that farming in Wales would "come effectively to an end" with the loss of £200m in EU subsidies.
Welsh Secretary Stephen Crabb accused Welsh Labour of making "wild, outlandish claims" on the threat to jobs, and claimed Lord Hain had been "resurrected as a ghost of Christmas past", years after he tried to "bounce" the UK into joining the Euro.
Mr Crabb said the Tories and the British people were in the "pragmatic centre ground".
"We are in government to bring some cool rationality about this," Mr Crabb said.
"The point that should be getting made about Wales's membership of Europe is about the value of trade - it's not about the subsidies it's actually the trading opportunities it creates.
"But those trading opportunities will continue to exist whether Wales, and Britain, remains in Europe or not."
UKIP policy director Mark Reckless, whose party wants the UK to leave the EU, accused Labour of wanting "27 other countries to make our laws for us because they don't think we're good enough".
"UKIP would like to trade freely with the whole world, not just with the one declining continent within it. You don't need to be governed by other countries in order to trade freely with them," he told BBC Radio Wales on Monday.
Recent opinion polls suggest slightly more voters in Wales think being part of the EU is a good thing, but a large proportion are still undecided.
Analysis by Nick Servini, BBC Wales political editor
Even by the standards of the usual pro-European rhetoric of Welsh Labour, this was a series of warnings designed to generate full-on fear about the dangers of pulling out of the EU.
At one stage the first minister even said that farming would effectively come to an end because of the removal of EU subsidies.
Their central argument is that Wales is uniquely vulnerable to a withdrawal because of the scale of EU aid received now and in the past. Former Welsh Secretary Peter Hain said the future of Wales was at stake.
The counter view is that a Westminster government would have more money to invest because it wouldn't be sending it to Brussels every year so Wales wouldn't lose out. There's no definitive answer so this debate will run and run.
The official Tory view on EU membership will rest on Prime Minister David Cameron's renegotiation, but it was interesting to note the current Welsh secretary Stephen Crabb wasn't afraid to strike a sceptical note about some of the Labour arguments.