The case for approving a £1.3bn tidal lagoon in Swansea Bay will be urged at a meeting with a UK government minister.
Plaid Cymru’s Simon Thomas and Conservative Russell George will press Business Secretary Greg Clark to back the green energy project on Thursday.
Mr Thomas said it could “kick start” coastal community regeneration.
The UK government said it was considering the findings of the Hendry review, which backed the plans.
Mr Thomas, who chairs the assembly’s finance committee and Mr George, chairman of the economy committee, will stress cross-party support in Wales for the plan, and the positive tone of the report published in January by former UK energy minister Charles Hendry.
“This would be an important pathfinder project which could lead to a whole new industry using the expertise devised in Wales,” said Mr Thomas, who is also Plaid’s spokesman on energy.
“It would be an extremely poor message to those involved in the Swansea Bay City Deal region if we lose a huge renewable investment project in the tidal lagoon due to dither and delay.”
Conservative AM Mr George said the delegation would show “a united front”.
He added the group “will be looking for assurances from the UK government as to when a decision will be taken on this transformational project”.
A spokesman for the Department for Business, Energy and Industrial Strategy said: “The government is considering the recommendations of the Hendry Review and taking the time needed to determine what is in the best interests of UK energy consumers and taxpayers in the long term.
“We will publish our response in due course.”