David Costa, who is a member of Welsh Labour’s executive committee, criticised the former minister on social network Bluesky.
He wrote the analysis “would be a lot more convincing had his ham-fisted presentation and implementation of the intrinsically sensible 20mph policy not sparked a hugely damaging populist backlash”.
“Democratic politics is about convincing others not just parading your own virtue.”
Rhun ap Iorwerth, Plaid Cymru leader, told a news conference the blog was a “realisation Labour is in a very very difficult situation”.
“We are seeing signs of people within Labour admitting that sense of inevitability of Labour leading government in Wales is no longer there.”
Asked if Plaid also risked being seen as the establishment in Wales, he added: “We know what the establishment is in Wales, and it has been Labour.”
In response a Welsh Labour spokesman said since becoming first minister Morgan has delivered “the largest financial settlement for Wales since devolution began, £50m to improve schools and college buildings, a £1bn investment in Shotton Mill alongside UK government colleagues, an extra £25m for coal tip safety, and free school lunches for every primary school child in Wales, and she’s only just getting started”.