it would decrease the power of large population cities which are usually high percent and total votes for democrats. Michigan, PA, CA and NY would be a good thing.
Somewhat. Congressional districts are apportioned by population (both between the states and within each state), so urban areas get more Congressional districts than rural.
Like I said in a post above, I am all in favor of reliably Democrat states doing it, since rather than all of their electoral votes going to the Democrat some will go to the Republican. On the other hand, it would be stupid for any Republican states to do it.