the meaning of 'alone'
The trade unions were no longer the constituency of the Labour Party alone.

What does 'alone' mean in this sentence? Does it mean that there were some other groups that supported the party or that the trade unions supported some parties other than the Labour Party?

Question

the meaning of 'alone'
The trade unions were no longer the constituency of the Labour Party alone.

What does 'alone' mean in this sentence? Does it mean that there were some other groups that supported the party or that the trade unions supported some parties other than the Labour Party?

Answer

It means that the trade unions were supported by parties other than the Labour Party.

The sentence could be rewritten as:
The trade unions were no longer just the constituency of the Labour Party.
The trade unions were no longer the constituency of only the Labour Party.

There is an unstated thought or fact which would be:
The trade unions were no longer the constituency of the Labour Party alone, they are now also the constituency of the XXXXX Party.

Answer

The trade unions were no longer the constituency of only the Labour Party.  They are a constituency of another party as well.
It means that not all the people in the trade unions vote for just the Labour Party anymore.