How plausible is this “COVID has driven decline in radical right” hypothesis? Let’s take just a simple look at the polling trends:
Germany: Rad rt decline began in early 2019, accelerated since Covid crisis began
Italy: Decline of LN began mid 2019 pre-Covid but https://twitter.com/epkaufm/status/1315258529092186112

...when far right FDI included no real rad rt decline at all
France: no real evidence of any decline in FN to date
Spain: not much evidence of decline in VOX tho it has stopped growing
Sweden: SD have fallen during COVID but only back to early 2019 levels

Netherlands: hard to interpret but seems tradition rad rt PVV have been rising since COVID began
Denmark: Rad rt collapsed in early 2019,pre-COVID
Finland: Rad rt PS down somewhat since COVID
Norway: sustained rad rt decline since 2018
Austria: sustained rad rt decline since 2018

U.K.: rad rt collapsed pre-COVID. Brief rise and fall of successor party in 2019 - also collapsed pre-COVID

Putting it together, we have 3 cases that might fit the hypothesis (incl 1 where decline was evident pre-Covid), 4 cases - all the hardest hit EU countries IT, SP, FR, NL - where there’s no evidence of decline, & 4 cases where decline is pre-COVID.

More comprehensive survey here finds similar lack of consistent pattern: https://twitter.com/mrooduijn/status/1298506616111796224

The case Kaufmann hangs this claim on - Austria, where the rad rt FPO just collapsed in Vienna elections - is also one where pre-COVID decline is clearest, and most readily explained by local factors - a corruption scandal involving the party leader & subsequent party split