That's a bit simplistic to call Putin's regime ultra nationlist. The key to Putin's power is that the Russian people see him as a strong capable leader, who let's them take holidays in Europe and they don't need to worry about security. Even with this invasion the Kremlin has been at pains to make the appearance of life for ordinary Russians to be business as usual. This is particularly true of the middle classes in St Petersburg and Moscow. His grip on power isn't the same as Stalin's was. The opinion of the Russian people do have a bearing on this. Much of the resources and funding that could have been spent on the war with Ukraine have gone on things like keeping the Russian air industry going so Russians can still fly to Cyprus on holiday or Switzerland for medical treatment, life for the elites and middle classes haven't really changed despite massive casualties and crippling sanctions. Now this has enraged certain nationalist elements in and around the government.
People like this chap
en.m.wikipedia.org
They believe that Russia should stop messing about and unleash everything on Ukraine and the west. Now they won't criticise Putin directly (although Pziogen did) but they feel that the corrupt elites of Russia are hiding from this war and are living their comfy lives while other Russians are dying fighting demonic, gay, Jewish Nazis.
These people want to "wake up Russians" and fully mobilize. Something Putin's regime absolutely doesn't want to happen.