Jamie Glackin hit out at party bosses in the wake of poor results in the European elections - where the party polled just 9% in Scotland and lost its two MEPs.
Scottish Labour leader Richard Leonard suffered another blow when two of his Holyrood front bench team quit their posts yesterday.
Neil Findlay, a close ally of Mr Leonard, stepped down as constitutional relations spokesman, with fellow MSP Daniel Johnson resigning from the justice brief.
Mr Glackin blamed UK leader Jeremy Corbyn’s “tin eared” approach on Brexit for the party’s woes, saying that while Labour had tried to appease Leave voters south of the border, this policy had cost votes in Scotland.
Writing in the Daily Record newspaper Mr Glackin said: “If I could pull the plug out of the Labour Party and restart it I would.
“We are essentially on life support at the moment and facing a terminal decline in Scotland.”
In the wake of the European election result, Mr Leonard said he “hoped” Scottish Labour would back a second Brexit referendum in all circumstances, and would campaign for remain.
This policy shift is to be voted on when Labour’s Scottish executive committee meets on June 8.
But Mr Glackin said while Labour had been in “bad situations before” - including when the party lost all but one of its Scottish MPs in the 2015 general election - things were now “much worse”.
He described the results of the European election as “cataclysmic”, and added: “Jeremy Corbyn and his inner circle have been tin-eared when it comes to Scotland.
“A substantial majority of Labour voters in Scotland, as elsewhere in the UK, were Remain voters. But that didn’t matter.
“Scottish Labour MPs have been among the most vocal in advocating for a People’s Vote, but that didn’t matter either.
“In trying to appeal to leave voters in Northern English constituencies, they have succeeded in driving the Labour vote share in Scotland to a cataclysmic 9%.
“The lesson we had to learn following the independence referendum was that we had to pick a side and stick to it. That side was obviously Remain.”
He also hit out at Mr Leonard saying while his predecessor Kezia Dugdale had won more autonomy from London for Scottish Labour, Mr Leonard had shown a “mystifying reluctance” to use this.