A poll conducted by the Electoral Reform Society at TUC conference about who delegates want to be the next Labour leader. Just under a third of the 700-ish delegates participated:
59% John McDonnell
10% Gordon Brown
8% Alan Johnson
rest undecided/other/not voting
This doesn't, of course, mean that McDonnell is even going to get on the ballot (though it can't hurt in terms of trying to get 44 MPs to nominate him), and nor is it representative of all trade unionists (delegates to TUC conference tend to be amongst the more leftie members in most union branches). But recent internal union elections have pointed to the strong organisation of the lefties and this poll certainly backs that up.
A third of the votes for the leadership get decided by the unions, and particularly if there isn't a high turnout amongst affiliated trade unionists, then this base of leftie organisers is significant.