Ed Miliband has said spending cuts to come in the wake of the Budget will be "devastating" for funding the Welsh government receives from the UK Treasury.
Speaking at Cardiff's Millennium Stadium on Thursday, he said Chancellor George Osborne "never had it so wrong".
Labour says Tory plans would see public spending return to 1930s levels.
Welsh Secretary, MP Stephen Crabb, contends the Budget will "help secure Wales' future".
Living standards, Mr Miliband predicted, would be "driven down for another five years" if the Conservatives have their way.
Labour, he added, would make "common sense spending reductions" while protecting key areas such as health and education.
Praising the Labour Welsh government's Jobs Growth Wales scheme, the Labour leader said it would be used as a model by his government for helping people into work.
Later, in an interview with BBC Wales, Mr Miliband said a Labour government would "look at" whether control of the Severn Bridges should be devolved to Wales.
Labour First Minister Carwyn Jones has called for his ministers to take control, promising they would seek to reduce toll charges that amount to "more or less a tax to come into Wales".
Mr Miliband said: "We"ll look at that issue.
"I think it's right that when the bridge comes back into public ownership, I think in 2018, we look at that issue of where control lies."
He said the party was "very, very happy" to look at the matter, adding that the first minister was making "an important case on that".
Mr Miliband said he knew the tolls were a "big imposition on people".
He also announced plans to set up a City Deal for Cardiff to help the Welsh economy grow and a Help to Buy ISA, which will affect 45,000 first-time buyers in Wales.