Unions in general should be in favor of amnesty or anything that makes it easier for immigrants to become naturalized because that repreesents a bigger pool of employees likely to be in unions.
The guest worker program doesn't offer that option.
Thes ILLEGALS, when granted amnesty, will not join traditional unions. They will form "la reza" (sp?) type unions that will only damage traditional US unions.--and, I'm guessing here, will have a lot more members than groups like Sweeney's.
Guest workers are only usfull for highly intensive, hard labor jobs. Sweeny's union is not really helped by that kind of influx. Most manufacturing jobs are now being done with high-tech robotics and sophisticated machines. (Well, the ones that aren't going out of the Country..) More jobs could go this way in the future, not less.
The Democrats get their strength by a coalition of ORGANIZED groups. Sometimes they do not all agree, but stick together in odd ways and hold their nose to get into power.