If Putin goes further into Ukraine, that's exactly what he faces. The divisions in Ukraine are not as cut and dried as they are often made out to be. He almost has to have Kherson, and Kheson is only a ~25% Russian speaking area.
There's also this from Elena Filatova's website, probably biased, but still worthy of note:
Now I shall say couple of words about Russian speaking Ukrainian who 40% of population in my country. Putin propaganda make people believe that Russian speaking Ukrainians all want to Russia. Please, don't believe this. Only bunch of paid Russian extremists want to Russia. They are often appear to be Russian citizens. In this battle is very important what side Russian speaking Ukrainians will take. Interesting fact is that 60% of officers in Soviet Union army were Ukrainians, mostly from Russian speaking part. Military was an occupation for Russian speaking Ukrainians for centuries, they lived on the border of Russian empire and served Tsar, or Turks or Poles or whoever paid them. War was normal state for them for centuries. They lived for war and they lived off war. What we see this days here in Ukraine is readiness of Russian speaking Ukrainians to fight Russian occupants. I wouldn't worry about Ukrainian speaking part. Everyone cool and reservists willingly join army. Spirit is high
http://www.angelfire.com/extreme4/kiddofspeed/maydan/updatemarch15.html
If only 30% of the Russian speakers support Ukraine's integrity, Putin has a big problem on his hands. We've seen in Iraq what happens when only a portion of the population is actively against an invading force. Plus, the "pro-Russian" regions are not 100% pro-Russian. It's usually 65% or something like that. Barring some sort of violent ethnic cleansing, a huge disaster for Putin itself, if that happens, the other 35% of the population is a big problem.
A very crude analogy on the language side of things would be Hispanics in the U.S. Sure, SOME Spanish speakers in New Mexico might like to see it become part of Mexico. But they are in the minority. Most realize they are better off in New Mexico, USA, than New Mexico, Mexico.
Correction: “occupying force”, not “invading force”.