The President is on the right track. I do think he should place a tax on the remittances Mexicans here send back to Mexico and there would be less noise from the opposition.
The President lacks the authority to implement a tax.
The remittances of which you speak are in reality no difference than routine cash transfers between Mexican and American banks or between say British and American banks.
Further, Although Western Union is the primary conduit, any Mexican and American can go into the business very easily. All it takes is a fax machine or now a days an e mail ability. I receive the cash. I advise my contact in Mexico, my cousin or uncle, he pays the designated receiver.
Once a month or so, I transmit all the collected money to my cousin and settle the account. It is very easy