45% of republicans according to one poll (https://www.independent.co.uk/news/world/americas/us-election-2020/republicans-congress-capitol-support-trump-b1783807

45% of republicans according to one poll (https://www.independent.co.uk/news/world/americas/us-election-2020/republicans-congress-capitol-support-trump-b1783807.html https://www.independent.co.uk/news/world/americas/us-electio...), 18% according to another (https://thehill.com/homenews/news/533452-poll-18-percent-of-republicans-support-capitol-riots https://thehill.com/homenews/news/533452-poll-18-percent-of-...). That's not all republicans but it's certainly many of them and probably enough to decide who the next presidential candidate will be. It's mainstream in the republican party, not some lunatic fringe.