About 400 people have protested outside the Senedd against the closure of an A&E department.
Cwm Taf Morgannwg health board have said doctor shortages meant it was considering closing the department at the Royal Glamorgan in Llantrisant, either completely or overnight.
Samantha Jones, whose baby's life was saved by staff at the A&E, said closing the department would put lives at risk.
The health board said action was needed to avoid "risk to patient safety".
An agreement to centralise emergency care in fewer hospitals was made in 2015, but a decision on the final details is yet to be made.
Assembly members from across the party divide who oppose the plans have warned people "will die" if the service closes.
But in the Senedd on Wednesday, Health Minister Vaughan Gething said they needed to "rub up against the reality" of staff recruitment problems.
"They do have an unavoidable challenge about the future safety of that service," he said.
However a majority of assembly members, including four from Labour, backed a Tory motion rejecting any downgrading.
Earlier, Welsh Conservative health spokeswoman Angela Burns said the Welsh Government could not ignore the "significant opposition" to the downgrading.
She said it would be "dangerous" to change the service as people would have to travel further for emergency treatment.
Staffing levels at all Cwm Taf Morgannwg's A&E units - at the Royal Glamorgan Hospital, Merthyr Tydfil's Prince Charles Hospital and the Princess of Wales Hospital in Bridgend - are well below UK-wide standards.
On Christmas Day and Boxing Day, ambulances had to be diverted from the Royal Glamorgan Hospital to Prince Charles Hospital because of a lack of doctors.
'I couldn't praise them more'
When Samantha Jones's daughter seemed unwell at home she took her to the Royal Glamorgan Hospital, which is about 20 minutes drive from her house, to get her checked over.
But when she got there the triage nurse noticed Scarlett had stopped breathing.
"Within minutes we had 20 doctors around us, surrounding her, with oxygen on her face," Ms Jones said.
"To go back over it is scary, but they did an amazing job.
"They did so many tests on her to rule everything out - I couldn't praise them more, they saved my child's life."
"If I had to go any further, I don't think she would be here today."
A number of politicians opposed to the plans addressed the crowds earlier, who held a protest ahead of an assembly debate over the future of the services.
Labour MP for Rhondda, Chris Bryant, said people "will die" if the department is closed.
He said: "We all know the only way to keep everyone across the whole patch safe is to have a full A&E at all three hospitals."
He accused the hospital of not recruiting properly, adding: "We in the Rhondda will never give up… we will fight and fight and fight for the service we know will save lives."
Plaid Cymru leader Adam Price told the crowd people were "dying unnecessarily because of the idiocy of the proposal".
"A people united will never be defeated, there is no power in this building or in the health board that will defeat us," he said.
The situation at the Royal Glamorgan worsened recently with the resignation of its only full-time A&E consultant, meaning plans to downgrade were speeded up.
The health board said this "expected retirement", along with a shortage of middle-grade doctors, meant three A&E services could not be "sustained beyond the immediate short-term".
It might seem odd to have Labour politicians joining in with the criticism of the decision to downgrade A&E at the Royal Glamorgan Hospital
"Doesn't Labour run the Welsh NHS after all?" their opponents say.
They know few things are more important to their constituents than protecting the services they have grown up with.
But it does not change the bigger picture. If you do not have enough doctors to run a safe service and cannot recruit them - what do you do?
Politicians across the board will privately tell you the NHS can't stand still and things have to change.
They would much prefer the most controversial changes happen somewhere else and not in their patch.
But trying to avoid the consequences of unpopular decisions risks a situation where nothing changes at all.
The nearest A&E to Llantrisant in the health board area is 14 miles (23km) away in Bridgend - a drive of almost 30 minutes.
The other in Merthyr Tydfil is 21 miles (34km) away - a drive of almost 40 minutes.
The health board has agreed to "leave no stone unturned" in seeing if anything could be done to keep the current A&E set-up as it is.
Labour AM for Pontypridd Mick Antoniw said there was no merit to the proposals and that closing the department was "not viable".
Plaid AM for Rhondda Leanne Wood attacked the Welsh Government for its record on health in Wales, adding the "strategic decisions" made in Cardiff Bay had led to the proposals to close the unit.