Illegal immigrants are defined thus (from here:
https://crsreports.congress.gov/product/pdf/R/R47218/3):
'This report covers several frequently asked questions about the
unauthorized immigrant population (sometimes referred to as undocumented migrants,
illegal migrants, or aliens in immigration law) residing in the United States.]
As regards their work status:
'In some cases, individuals with certain statuses or pending applications may be eligible for work authorization.'
I'd assume that if you were authorized to work then you'd be free to join a union.
And Trump intends deporting 'unauthorised immigrants', which is the same, as we have just seen, as saying illegal immigrants. From here:
Trump touts historic deportation plans, but his own record reveals big obstacles
'Trump himself has talked about enlisting local law enforcement and National Guard troops to extend ICE's reach, while some of his allies have even floated the idea of "staging areas" or detention camps near the southern border that would allow the administration to arrest, detain and deport unauthorized immigrants by the millions.
But some immigration analysts and former ICE officials say the Trump campaign's goal of deporting many of the roughly
11 million unauthorized immigrants in the U.S. will be expensive and logistically challenging — if it is feasible at all.'
See that figure? Eleven
million? That's the population of Ohio. It's the
combined population of Chicago, Houston, Phoenix and Philadelphia.