Crimea was traditionally part of Russia; ethnic Russians are a large majority of the population and Russia has major naval bases there. Crimea was ‘given” to Ukraine by N. Khruxhchev, himself Ukrainian. If Putin intends to take Crimea by force, it will be extremely popular with most Russians, except for the Crimean Tatars whom Stalin deported after the war for collaboration with the German Army. I’m not taking Putin’s position, but I understand it.
Khrushchev was not an ethnic Ukrainian, but he did serve as Boss of the Ukrainian SSR, under Stalin. So he felt giving Ukraine the Crimea would win over those Ukrainians who didn’t care for his rule there.