Why would I ever join a union that didn't protect me from rampant undercutting via importing more workers? That is the whole reason I would join a union.
I understand that you want to retaliate against immigrants and people from other countries.
But if that is your opinion then don't expect anyone who has an H1B visa to ever agree with you.
You should probably expect people who don't hate immigrants to fight against your efforts though, and to take actions to sabotage the union.
Don't expect to ever get those other people on your side, if your argument is that you just don't want foreign workers to be able to get jobs here.