Legal immigration is fine. Illegal immigration runs the risk of defective political systems immigrating as well. And being adopted by our political class. Numerous links to supporting articles at the source.
A former Mexican President is on record of a trial informant as having received a $250 million bribe from the dominant, Narcotrafficantes Sector of the Mexican Economy, he asked for $500. Our Banana Republic, 800 pound gorilla is the Koch Bros., Warren Buffett/Berkshire Hathaway, the U.S. Chamber of Commerce and the Business Roundtable, among others, who use the century old Opinion Engineering industry to get college graduates, the audience for the PBS Newshour, to feel agonized compassion for the legions of undocumented migrants, meanwhile there is no "compassion" for poor African-Americans who are displaced by the migrants. These are the people holding the strings of your former great American elections.
While this is a long read, there is excellent information here.
Everything from forging new precinct totals to outright data manipulation, this paper reveals the various ways that voter fraud is perpetrated.
My personal favorite: the manipulation of vote tallies in the Second District of Puebla:
An official would page through the pile of precinct tallies one by one, calling out in a loud voice—in Spanish, cantando—the votes for each candidate as a secretary wrote the totals onto the district spreadsheet. (…) Each time Salinas’s votes from a precinct were read out loud, the PAN representative complained, the district committee secretary was adding a zero to Salinas’s total on the spreadsheet, changing 73 votes for Salinas to 730 votes, for instance.
And how is this non-computerized voter fraud any different than what we see today, except that the Dems have introduced multiple insertion targets into the voter count?