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."
Click here for more information on the constituency and a full list of candidates.

Tevez, who missed a penalty against Wigan last Saturday, was back on the bench for the midweek Champions League game against Napoli.
Mancini insists that Tevez, who failed to secure a summer move away from the club, must accept the situation.
"In this moment there are players that are better than him but Carlos is here training," he said.
"Carlos will have a chance to play. I don't know if he is happy but every player that can't play is not happy. This is normal.
"He will have his moment."
The form of fellow strikers Edin Dzeko and Sergio Aguero, who have scored six league goals apiece so far this season, mean that the 27-year-old Argentina international's chances have been limited, while Mario Balotelli has also played just one league game.
But Mancini believes his free-scoring side still need to be more ruthless in front of goal than neighbours and arch-rivals Manchester United, who have scored 18 goals - three more than City - in their first four Premier League games.
When City underlined their title credentials with a 5-1 win at Tottenham last month, United bettered them with a 8-2 victory over Arsenal.
Mancini's men impressed again by beating Wigan 3-0 last week, only for United to hammer Bolton 5-0.
The Italian said: "Man United are a top team - they have been used to winning for a long time. They have strong players.
"When they played against Arsenal, when they scored two or three goals, they continued trying to score.
"We need to have this mentality. If we have a game like Tottenham where we can score, we need to continue to try to play."

The fight, Adams' second as a professional and first in her home city for 20 years, will be over four three-minute rounds, rather than the usual two-minute rounds for women.
The flyweight voiced her frustration with two-minute rounds after beating Virginia Carcamo last month.
Salazar, 18, has lost once in six professional contests.
Adams, 34, says the extra minute in each round will give her a chance to try out things she has learnt in the gym.
She said: "I had two minutes to find my rhythm, distance and take out my opponent. It's not enough time.
"I found, by the end of the rounds, I needed a couple more seconds and it would be over.
"Now we have three minutes, I'm able to relax more, establish the jab, find the rhythm properly and really get in the swing of things."
On the same card, WBC international featherweight champion Josh Warrington defends his title against Spain's Kiko Martinez.
The Prime Minister has given his public backing to a stadium in Cornwall if the Conservatives are re-elected in May.
[NEXT_CONCEPT]
Manchester City manager Roberto Mancini says Carlos Tevez must wait for his chance to get back into the team.
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Britain's double Olympic champion Nicola Adams will face Mexico's Maryan Salazar in Leeds on Saturday.