These are the Brexit vote map, a map showing deprivation in England (dark blue is more deprived), a map showing the 2016 US election, and a map of income in the US (dark green is better off)
How do you fight right-wing extremism?
Fight poverty

To me it is clear how Biden and Starmer should run their campaigns
Offer change, not fear
Saying that they will return to the normality of 2015 politics is the opposite of what Brexiteers and Trump supporters want

Places like Clacton, Essex have huge levels of unemployment because of a lack of good government investment
Pennsylvania went Blue in every election since 1988 until 2016 because Trump promised small town voters a return to the height of steel production

Poverty leads to resentment which leads to far-right politics
Don't shout at the average Brexit or Trump voter for their politics, shout at the system and the conservatives who have propped it up
The best way to fight this political trend is to help poorer communities