Hartlepool was one of the most Leave-voting constituencies in the UK in the referendum. It only didn't go Tory for the last two elections because the Brexit Party and before that UKIP split the right wing vote.
Personally, I think Labour needs to represent remainers (48% of the electorate) more effectively. OK, the UK voted to leave and we have, but many many people voted against it and it was a narrow majority.
The Tories are winning in seats like Hartlepool by pretending everyone voted to leave, which we didn't, and enacting a very hard Brexit. Sending out 'gunboats' against French fishermen on the day of the local elections - did most people want shows of force against our closest neighbours to come out of the referendum? I don't think so.
I think there's a lot to be said, politically, about how Brexit is not going that well, and what could be done to improve the deal, but Labour has stayed silent on that. I think we're already close to seeing some UK concessions on SPS and on fish –and that's the kind of point the Labour Party should be making. Here in the English left-leaning cities, the protest vote is Green, not Tory – though I think Labour will hang on in most of them. But the message that's intended is that we can't keep pandering to a hard Brexit and to Tory donors –we need to look ahead to a fair transition in the light of climate change and to making the Brexit deal work for everyone, not just leavers, and to funding public services properly and fairly.
I think some of the answers as to who the Labour Party should now be targeting can be found in these maps of the referendum result: www.bbc.co.uk/news/uk-politics-36616028