I'm sorry, I'm a bit confused. What do you mean by 'demographics'? That is to say, my understanding of the word 'demographics' of a nation is a series of identity classifications of the citizens and non-citizen residents of the country such as: African-Americans, Indigenous-Americans (erroneously called "Indians" once upon a time), WASPs (standard, somewhat general White Anglo-Saxon Protestants), and all the people of all the composing ethnic, religious, and national origin groupings who reside in the United States; the two genders, men and women are also 'demographic' categories, as I understand the term 'demographics,' as well as people who consider themselves transgendered.
We must include sexual orientation, homosexual, heterosexual, and bisexual as 'demographic' categories. As I understand the matter, demographic categories also include region of the country where people live. And demographics might include educational level, class, and job status (white collar, pink collar, and blue collar). And so on and so forth.
Now, let us return to the question: How have labor unions impacted the demographics of the USA? Well, in the United States, as opposed to Western Europe and Canada, I would have to say that labor unions have not had the impact on the lives of the demographic groups of the United States. * By the way, we have to distinguish between labor unions and "the labor movement," which are two different things. The labor movement (along with the Progressive Movement of the early 20th century) gave us the eight-hour work day, weekends off, and so forth.
Labor unions in Canada and Europe tend to ask for benefits for the whole society rather than for their own narrow working union group. For example, one reason we do not have universal healthcare in the United States, is because the labor unions advocated for benefits for themselves only; in Canada and Europe, the labor unions insisted on healthcare for EVERYBODY -- this made it a hell of a lot harder for employers to play one group off against another.
There's more I could say... but I won't.
Take it easy.