You don't have to be a citizen. You just have to have a green card, but they won't sponsor for your greencard. I have tried for over a year and now I gave up as literally every single airline HR people told me that they will never sponsor me. In fact I had pretty close connection to 3 regionals via their company cheif pilots and even they couldn't get the HR to help me out with it.
Those Europeans people are talking about must have green card. If you have a green card you are all set, but if you don't have greencard than my experience says you have no chance. In fact an HR lady at a regional yelled at me for wasting her time when I told her that I have a work permit and I need her help to transfer it to their company.
But of course you never know. Sometimes they hire guys who shouldn't even drive cars and sometimes they turn down guys that should get the job before anybody else.
I have a work permit. I got called by every single regionals for interview except Skywest and Expressjet. Only PSA even give me the chance to interview. The sais bye as soon as I said I have a work permit.