FORMER cabinet minister Lindsay Tanner has warned that Labor could be entering a time of ''unprecedented bleakness'', in a swingeing critique describing the party as ''an electoral machine largely devoid of wider purpose''.

Mr Tanner slates the Gillard government's agenda as being driven by political circumstances and improvisation, and says the party needs a ''complete root-and-branch rethink about why we exist'', in his new book Politics with Purpose released today.

Finance minister until he quit at the 2010 election, Mr Tanner says Labor governments ''still do good things, but at the behest of random external forces, not any kind of inner calling''.

While it is possible to make a case that the agenda of the federal government is driven by great Labor purposes, ''closer examination reveals a harsher truth'', he writes, in criticisms that undermine the Gillard government's claims to be pursuing Labor values.