Much recent commentary on the conservative side addresses a perceived threat from potential illegal voting by non-citizen and/or undocumented immigrants.  As just a couple out of many examples, this article in the Washington Times back in February reported on a lawsuit by the Public Interest Legal Foundation that claimed that some 100,000 non-citizen immigrants were registered to vote just in the state of Pennsylvania; and this article at Fox News just today by former House Speaker Newt Gingrich reports on a plan in San Francisco to allow non-citizens to vote in local elections, allegedly as a precursor to getting them to vote in state and federal elections as well:

[T]he long-range plans Democrats have for a ruling majority depend on continuous law-breaking to get enough non-Americans to vote. The Californians who don’t support the radical views of Democrats can simply be eclipsed by non-citizen voters supporting the Democrats.

Now, I don't claim to know how many non-citizen and undocumented immigrants have been voting illegally in U.S. elections.  Some on the right claim such voting is a major problem, while many on the left pooh-pooh that idea.  But here's what I do know:  At least as it concerns the balance of power in Congress, it really doesn't matter much whether non-citizen and undocumented immigrants vote or not.  Non-citizen immigrants affect the balance of power by their mere presence.  The only factor that counts in the equation is whether the immigrants concentrate themselves in Democrat-voting districts -- which they do.  Needless to say, the leaders of the Democratic party are completely aware of this dynamic.  Without doubt, this is a big part of the explanation for why Democratic party leaders are such fans of expanded immigration and/or open borders, even though large flows of low-skill newcomers would seem to disadvantage the traditional Democratic base by providing substantial wage competition to low-wage workers. 

Does this whole thing seem counter-intuitive?  Let me explain.

The basic starting principle is that Congressional districts are apportioned based on "population," not based on numbers of voters.  The Constitution (Article I, Section 3) calls for what it describes as an "actual enumeration."  Maybe that could be interpreted different ways, but the practice, at least for many recent decades, has been to count everybody, irrespective of voter or citizenship status.  If you start thinking about it, at first it seems obvious.  Do we count children?  They don't vote, but of course we count them!  How about prisoners -- who in most cases are deprived of the right to vote?  They are also counted, in the place where they are incarcerated.

When you get to immigrants, it becomes less obvious.  According to information from the Migration Policy Institute (from 2016), there are about 44 million immigrants living in the U.S. -- 13% of the population.  Of those, something over 40% (or perhaps 18 million) are citizens, and around 11 million are undocumented (or "illegal').  That leaves another 15 million who are here legally but are not citizens.  That last group includes everything from students here on temporary visas all the way up through permanent legal residents (green card holders).  In the Census, and for the Congressional apportionment, all of these people get counted -- the citizens, the legal non-citizens, and the completely illegal non-citizens.

And by the way, it's not like there's any doubt about this proposition.  In 2016 a case called Evenwel v. Abbott went to the Supreme Court, specifically challenging whether non-citizen and illegal immigrants should be counted for purposes of legislative apportionment (both state and federal).  The Supreme Court, in a unanimous opinion, ruled that "population" controls, not numbers of voters.

It then turns out that immigrants concentrate themselves in certain areas.  Many immigrants from all parts of the world concentrate themselves in the major coastal cities.  Hispanic immigrants from Latin America concentrate themselves in areas in states like California, New York and Texas that already have large numbers of Hispanic immigrants.  Not all, but the large majority, of these areas send Democrats to Congress.

At this link, a guy named Adam Griffin has compiled information by Congressional district of the ratio of registered voters to total population.  The ratios range from about 30% at the low end to over 65% at the high end.  As examples, the three most southerly districts in California have ratios of 33, 38 and 46.  The three districts in Texas at the south end of the Rio Grande valley have ratios of 36, 40 and 41.  While the correlation is not perfect, the more Republican a district leans, the higher its ratio of registered voters to population tends to be.  In North Dakota, the ratio is 76%.  (That may reflect an aging population -- lack of children -- as well as dearth of immigrants.)

Granted, the effect of this phenomenon only registers with the decennial census, and nothing about new immigration this year is going to affect the apportionment for the 2018 or 2020 elections.  Nevertheless, the overall effect is that Democrats get to represent in Congress something in the range of 15 to 20 million non-citizen immigrants, without those immigrants ever needing to vote.  As a rough approximation, this represents about 20 or so seats in Congress, and it could even go up somewhat after the next apportionment.  This swing dwarfs any possible effect of actual illegal voting.