Consequences of quitting a graduate-level Research Assistant (RA) position mid-semester, with little notice? BACKGROUND

I am an engineering graduate student at a good American university, funded as a Research Assistant (RA).  The lab and professor rely heavily on my day-to-day work.  

My PhD research is going nowhere and I am far less interested in obtaining a PhD compared to one year ago.  Most importantly, my wife wants me to leave and pursue an engineering job with a salary.  I have a colleague/friend who has a position lined up for me provided I begin to work ASAP.  Ultimately, this would mean quitting my funded research position mid summer (and mid-semester) with little notice.  Because of this, I am not expecting a "letter of recommendation" from my advisor/boss/professor, however I don't want to be blacklisted either.

QUESTIONS

How significant of an offense would it be to quit a graduate-level Research Assistantship in the middle of a semester?  Would it even be legal to do this at an American university?  What would be the consequences for simply turning in my resignation and leaving the program (again, in the middle of the semester with very little notice)?

Opinions from professors, PhD drop-outs, or people familiar with these situations would be great!
This will likely vary greatly from one department to another. It will also depend upon your relationship with your advisor, departmental needs, etc. As @tonysdg noted in the comments, it would be a good idea to talk to your advisor about this. Some departments are understanding of these situations.

In my (geography) department, a few years ago we had a student quit his masters degree and his teaching assistantship because he was a single dad and out of money. He dreaded going to his supervisor (who was not his academic advisor) to tell him this. But it ended up being no big deal. Everyone understood, and to my knowledge there were no harsh feelings in the department toward him. Other grad students and professors covered a few classes for him through the rest of the semester, and the department was able to fund a new student the next semester.

Like I said, these things are situational, so your conversation may be different. But I included this answer as a tangible example that it is possible to leave in the middle of the semester without causing bedlam. Also, there may be strict rules against leaving in the middle of the semester, but sometimes these rules can be bent under the right circumstances. Even if you find a rule against what you're thinking about doing, I would still talk to your advisor.