1) The biggest proof is under Biden's last term and at the end of Obama's last term, there were very large numbers of illegal border crossings. Especially under Biden where millions crossed. Let's conservatively say it was seven million people in four years. That doesn't happen by accident. Why do I say that? Under Trump's terms the border crossing numbers dramatically diminished. So it was just a question of enforcement which means the DHS, which happened to be run by the same guy Alejandro Mayorkas under both Biden and Obama, simply chose not to enforce the border and allow very large numbers of asylum claims (which I think are bogus and not our responsibility). That's a very nasty thing to do. Note that neither Biden nor Obama openly campaigned on letting millions of people in and it was a big concern of voters going into the 2024 elections.
2) Democrats have run on a racial platform and it has been very successful. See: https://keywiki.org/Race_Will_Win_the_Race. For example, California used to be a GOP state. Ronald Reagan was the governor of California. The demographics of California have changed dramatically in just a few decades as has the politics. https://en.wikipedia.org/wiki/Demographics_of_California#202...
3) Democrats have refused to comply with ICE. ICE is responsible for deportations. They have created sanctuary cities, counties, and states. What reason is there for this besides not wanting illegals to be removed? "Human rights" is just a way of saying "endless litigation to prevent deportations".
4) The census was changed under Biden to include illegals in the population count. This is used to apportion representatives. So that would give more representation in the House to Democrats since illegals tend to live in urban areas which are all represented by Democrats.
5) Democrats have repeatedly made claims about how diversity is our strength. If diversity is to be valued, they must favor more immigration. Diversity here seems to be a euphemism. No one calls for diversity in China or Saudi Arabia for example. Why not? I think we know. So immigration and diversity are seemingly euphemisms for something else. https://www.c-span.org/clip/white-house-event/user-clip-joe-...
6) The 1965 Immigration Act signed and sponsored under Democrats. https://en.wikipedia.org/wiki/Immigration_and_Nationality_Ac.... It called for less homogeneity of immigrants. It has caused demographic change. If we look at how ethnic groups vote, demographic change has primarily benefited Democrats.
I think it's just obvious also. Lots of political things are put in terms of euphemisms that use watered down language. But it's quite hard not to notice ethnic conflict and balkanization beginning in the US and even across Europe.