Welsh Secretary Alun Cairns has told MPs he would "really like" to see the Swansea tidal lagoon go ahead, as an expected announcement is postponed.
Supporters fear the UK government is on the point of rejecting the £1.3bn energy project on grounds of cost.
Mr Cairns said the lagoon's backers must prove it is value for money.
During heated Welsh Questions, Plaid Cymru MP Jonathan Edwards said he would be known as the "grim reaper of Welsh politics - the bearer of bad news".
The project was backed in January 2017 by a UK government-commissioned report published by former energy minister Charles Hendry, which recommended that tidal lagoons could play "a cost-effective role in the UK's energy mix".
But ministers in the UK government have refused to commit to the project put forward by Tidal Lagoon Power (TLP) saying it "must be affordable".
Amid press reports that the proposal would be thrown out, an expected announcement by Business Secretary Greg Clark on Wednesday was postponed.
Repeatedly pressed on the matter at Welsh Questions in the Commons, Mr Cairns said: "I would really like the tidal lagoon to go ahead but of course it must prove to be of value for money."
Swansea East Labour MP Carolyn Harris asked when he would "start speaking up for the people of Wales", pointing to the cancellation of rail electrification between Swansea and Cardiff as well as doubts over the lagoon scheme.
Mr Cairns said: "We're still looking at the numbers - we're doing anything and everything possible to try to make this fit."
Plaid Cymru MP Jonathan Edwards said: "When he pulls the plug on the lagoon there will be huge public anger in Wales," adding that they would ask what the point would be of having a Welsh secretary.
Mr Cairns rejected the criticism, pointing to the scrapping of the Severn Bridge tolls plus backing for city and growth deals as proof of the value of his office.
Last week, it emerged that Mr Cairns had queried the cost of the energy produced in an email, saying that although he was a supporter, it looked "twice the price of nuclear" and offered a fraction of the jobs.
This prompted First Minister Carwyn Jones to renew his offer of financial support from the Welsh Government, with the figure of £200m made public for the first time.