Putin cannot afford to lose a war or to lose Russian territory. Putin is only president for life. No Russian leader has personally survived such an event, all the way back to the tsars. Faced with that history, he is more likely to use a tactical nuclear weapon than give up power.
Alexander I survived losing the wars of the third and fourth coalitions against Napoleon, Alexander II survived losing the Crimean war, Kerensky survived losing WWI, Lenin survived losing the Polish-Soviet war, Gorbachev survived losing the Soviet-Afghan war, Yeltsin survived losing the Chechen war...
What many forget, or perhaps nver realised, is that Putin is the moderate voice of the extreme Russian nationalists.
Should Putin lose his hold on power to any degree he could be usurped by anyone of several who would have absolutely used nuclear weapons three months ago already.