Leftists of course deny the fact since it serves their purposes to do so. Their grand strategy is to win by demographic means.
Truth is not a leftist value. Or rather, it is if it supports their agenda, but is not if it doesn't.
For a leftist, the difference between a citizen and a non-citizen is the difference between someone who is physically present within a nation's boundaries and one who isn't.
John Fund has done good, objective work on voter fraud. See here and here.