Russians have tactical nukes. A few would severely stop much of the forces. Don’t think they wouldn’t do it with collateral damage to innocent citizens. Our government would. They are worse and harder than us. And less encumbered by laws to do it.
But Putin wouldn’t be able to launch the bomb himself. A dozen different men named Boris with vodka bottles, all with family or distant family in the region, would have to be part of the machinery that deploys and launches a nuke or even just a regular bombing campaign, the latter of which Russians suck at since their usual tactic is to do indiscriminate bombing, not precision bombing. That works fine in an invasion, not so much for internal strikes.