Is there any proof for this statement? Republicans want cheap labor, they'll tell you that themselves, but I don't believe the Democrats would ever say that they believe in labor immigration to "get more voters".
That just sounds like a ludicrous conspiracy.
"Demographics is destiny" seems to be a quote from a french philosopher from the 1800s, while "coalition of the ascendant" seems to be a description of the coalition Obama sought to build in his second term, an idea swiftly dismantled in 2016.
The articles I can find discussing these things don't contain any democrats scheming about using immigrants to overtake the republicans. The only people I find talking about how it will "estrange the republican party from these growing demographics" are republicans. I'm not going to claim that no democrat has ever considered that outcome, but I am going to propose that it seems more likely to me that if that were to happen it would be a result of politics they'd be in favor of anyway.
In general, it's pretty popular to help people. I don't think republicans would disagree that part of their coalition is built on higher income individuals, which they hope to grow by raising incomes. They're not doing that as a sort of underhanded tactic, they believe income is good. In the same way I think the Obama democrats do believe in helping immigrants, and obviously that makes them more popular amongst immigrants. That's not a scheme, that's just a policy.
I don't think this is nearly as clear cut as you believe it is.