Follow the money. Unions, dems and big business all behind it. Loss of sovereignty, US safety, culture, laws, mores, language mean nothing compared to what they think they'll get. Although, when they're impacted by an illegal who drives the wrong way on the highway, killing their children...or they don't get a job because illegals are being paid coolie wages...or Mexican gangs run wild...drugs, murder, you name it, they might spare a passing thought for what they threw away.
Not all unions are supporting the Senate bill. http://www.local2544.org/