Republicans lost seats because the party in power loses seats in every midterm election.
Under Barack Obama,
The Democrats got their butt kicked, and under
Donald Trump, Republicans also lost a lot of seats. It's nothing new. Non-citizens cannot vote, period.
There HAS been legislation proposed that would allow non-citizens (
not necessarily illegal immigrants, but any type of U.S. visa holder or foreign national) to vote, but it has only ever been for local elections — city elections at that, so something much lower than federal. Non-citizens cannot vote for president, or U.S. legislature.
In addition, there actually are places that allow non-citizens of that country to vote, namely if they're a citizen of a collective union like the EU or the Commonwealth of Nations (former British colonies and the United Kingdom proper). But the United States is not one of them. It seems to have allowed it on a state by state basis, which each state being the arbiter of whether voting rights are for full citizens or just people who've lived there long enough to establish residency. By
1996, this changed nationally.