>>In NY and NJ, it's impossible to vote twice.
>>Bill Fitz registers and has one line on the elector roll.
>>When he shows up to vote, he signs the roll.
>>If Bill Fitz shows up again, the electors (one from each party) says "Hey, Bill, you voted already- there's your signature"

Bill turns up to a different polling booth, or pops across to a swing state where he used to live.
Bill votes again.

>>Registration is another matter.
>>NY and NJ now require some kind of proof of residence or identity. I might just be a social security number.
>>Sure, I can probably figure out a way to register as Mike Fitz and double my vote.
>>Do you really think that that's a feasible way to swing an election?

Focusing on California:
Illegal immigrant Bill gets a driver license.
Hey Presto! In his district, a CA license means Bill now can vote.
So he does.
But California officials insist there's no possibility of voter fraud, even calling Trump a liar.
Meanwhile, CA does not cross off names of voters who no longer are eligible (e.g. because they died or moved out of state,) instead sending absentee ballots every time to where they used to live.
Some of the deceased proceed to vote.

The deceased also get to vote in New York: http://nypost.com/2013/12/30/the-dead-can-vote-in-nyc/

Florida: see http://6889-presscdn-0-68.pagely.netdna-cdn.com/wp-content/uploads/2016/11/Chelsey-Smith-SOE-Affidavit.pdf

Philadelphia: http://www.theepochtimes.com/n3/2181836-report-state-police-raid-offices-of-philadelphia-grassroots-voting-organization/

AFAICS the risk of voter fraud is very real. Nevertheless we're instructed that voter fraud has been disproven zillions of times and the MSM, predictably, calls it yet another self-serving/egotistical/(add epithet du jour here) misleading claim by Trump to be added to the 585 others.

Voting is such a fundamental part of any democratic process that this slackness and attempts to perpetuate it, are anti-democratic. Trump's investigation is overdue and those clamoring to oppose it, are the ones whose motives ought to be reviewed.