[A CA Guy]

The left promote a lot of employment through the government. The unions from that finance the Democrats and gives the Democrats a large percentage of the vote every election because the government employees are voting their wallets.
So those that vote with the left are mostly often entangled with them.

[NZerFromHK]

It is the same case with New Zealand. The Labour Party's biggest backers, as the name suggests, are labour unions. Basically they support whoever Labour politicians.

Curiously in Canada the unions largely support the far left NDP while the centre-left Liberal Party is not as widely supported. Nevermind, even without the unions the Canadian Liberal Party will still be returned to power given the left-leaning sentiment that permeates Canada.