However the real reason Democrats support and defend the illegal aliens is because they are easy to control and manipulate. The very type of individual that makes for a reliable Democrat voter. Most illegal aliens have no concept of our country except how easy it is to get free stuff and how to evade the penalties for breaking our laws. 
Democrats have proven time and time again they do not want any immigration law enforced, they would be more than happy to see open borders with no immigration laws at all. The Democrat understand their only hope of maintaining any political relevance is to import more and more uneducated and misinformed legal immigrants, and illegal aliens.