It's going to get massively less popular once a lot of those people realise the benefit he's cutting is the one they get.
No, it isn't, or at least it would be valid if he said 'Labour is now not the party of working people' but he didn't, he claimed that title for the Tory party while effectively doing nothing to help 'working people'. Changing the benefit rules should hardly affect 'working people' at all, because in theory we should only be giving benefits to people who aren't earning enough to live on and everyone in a job should earn enough to live on. The problem is the UK has become such a screwed up place (and it's both a Tory
and labour generated problem) that a lot of benefits are actually paid to 'working people' in the form of tax credits, child allowance and etc and they're what's keeping their head above water. The party of 'working people' would be sorting that situation out before cutting the benefits some of those 'working people' rely on. But they're doing it the other way around.
It boils down to this - he's making life worse for some working people, making life actually no better for the rest of the working people and saying that's still good for them because he's making life much, much worse for another group who are nothing to do with working people. If you can figure out how that allows them to claim to be 'the party of working people', well, good luck to you. He's basically saying that making it worse for other people of itself makes it better for you. You may subscribe to that view, but it's quite unrealistic.
This is completely aside from the point that it's pretty laughable for a man who has never worked a proper day in his life, who heads up a group of people quite a few of whome have also never had to work a day in their lives, claiming to represent 'working people' but anyway...