Central Scotland MSP is odds on favourite to replace Kezia Dugdale.
He said the rest of the party must 'fit in' with Jeremy Corbyn.
Jeremy Corbyn-ally backs centrist in sign contest north of the border could be close.
HuffPost reveals 2,700 members signed up - in just one weekend.
Cash-and-carry firm Anas Sarwar has shares in does not pay the real living wage.
Race will be a litmus test for the strength of Scottish Labour's left wing.