Farage's party badly defeated after attempting to exploit Rotherham child abuse scandal.
"I will fight with you with every fibre of my being over these months to show how we can change Scotland," says the Labour leader.
The shadow cabinet is likely his next stop.
"The folk who say you can only do this from the left aren't following the lessons of recent Scottish electoral history," says the Scottish Labour leadership frontrunner.
Even those inured to the gang warfare that periodically engulfs the party are stunned by the events of the last week.
Labour should be prepared to go in to coalition to prioritise strategic investment, a modern constitutional settlement and fairer austerity measures in government.
The former minister warns his party to argue in favour of the EU "much more than we have done", and to avoid “chasing a Ukip vote”.
The departure of the party's leader leaves it without an immediately obvious successor.
The Unite head's comments suggest the shadow health secretary could win his support in a future leadership contest.
The Labour leader aims to position his party as the only one offering "credible" change.