The country’s largest trade union backed Richard Leonard after a torrid week for the Scottish Labour leader.
Four of his own MSPs and several senior Labour figures have called for his resignation amid dismal polling results.
But Unite Scottish secretary Pat Rafferty said: “Voters will be repelled by this sort of conduct.
“They want to see their MSPs focused on supporting communities and saving jobs, not indulging in underhand efforts to dislodged an elected leader. Whoever is organising this disgraceful coup should pack it in at once.”
Meanwhile, Labour former First Minister Henry McLeish said the party should not be looking for a new leader just eight months before the next Holyrood election.
Mr McLeish said: “The Labour Party is fooling itself if it thinks a change of leadership will change the party’s fortunes. It’s much deeper than that.
“The public don’t know what Labour stands for. Unless we get to grips with the constitutional question, Scots will continue to ignore us.
“It is time for the party to think about the bigger issues we face – our attitude to Brexit, Scottish independence and offering an alternative through federalism.”
He added: “I have seen nothing yet to convince me that changing the leader will change the party’s prospects in next year’s election.
“If there’s some remarkable person out there who can do all of that, I haven’t seen them yet.”
Mr Leonard urged his party to unite behind him after UK Labour shadow cabinet member Rachel Reeves called on him to “consider his position”.
But former Labour government minister Brian Wilson said demands for loyalty from a besieged leader “should be treated with suspicion”.
Mr Wilson, writing in The Scotsman, said: “In last year’s European elections, under Corbyn and Leonard, Labour won 4% of the vote and under 10% in Scotland as a whole. Four per cent, Richard. Is that what I am supposed to be loyal to?”
Mr Leonard last night said he was determined to lead the party into next year’s Scottish Parliament election.
He said: “The events of the last week have only served to strengthen my own resolve and bolstered the confidence of party members in Scotland that we cannot go back, that we have to go forward together in the fight of our lives for every vote and every seat in next year’s election.”
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