David Cameron said a stadium would "inspire young people, brings jobs and wealth" to the county.
Labour said it was a "cynical" attempt to distract voters, the Liberal Democrats questioned where the funds would come from, and Mebyon Kernow said it did not believe the Tory's promise.
Recent planning rows have left the future of the stadium in question.
In March, Cornwall councillors deferred a decision to grant permission to build a supermarket that would have paid for the facility, which supporters claimed had put the plans in jeopardy.
Revised proposals for the stadium at West Langarth are expected to be submitted to the council in May.
Mr Cameron said he planned to get the Department of Culture, Media and Sport to work with Sport England, Cornwall Council and local sports teams to find a way to get it built.
"The stadium proposal is an exciting one. If it takes some extra money I'd make that money available," he added.
However, Stuart Roden, Labour candidate for Truro and Falmouth, said: "This announcement today is a cynical empty promise and will come to nothing.
"It is simply more smoke and mirrors and people won't be taken for fools by this unfunded distraction."
Simon Rix, who is standing for the Lib Dems in the constituency, told BBC News: "I'm massively in favour of a stadium in the right place if we can get the money.
"But it's like the Conservative promises on the NHS, they're not saying where the money's going to come from."
Stephen Richardson, who is standing for Mebyon Kernow in Truro and Falmouth, said: "You have to excuse me if I don't believe a single syllable of any David Cameron promise during the election campaign."
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The Prime Minister has given his public backing to a stadium in Cornwall if the Conservatives are re-elected in May.