Have you actually researched it? I'm going to rely on the historical record easily backing me up on this. For starters: https://www.democracydocket.com/analysis/ten-voting-rights-cases-that-shaped-history/
Those are just a handful of Supreme Court cases.
And I added that disclosure because I want to show that I'm not blind to the politics of it. Presumably you agree that, regardless of who they will vote for, American citizens should not be denied the right to vote because they are poor.
What's your basis for claiming that "illegitimate voters favor Democrats?" I know people make that claim in the media, but I have yet to see any legitimate research that supports it. If you know of some, please share.
@Honestbuckeye covered it.
The argument against requiring ID is so ridiculously weak that it is and can only possibly be cover for the real motivation.
I also do not trust the State of California AT ALL to let non-citizens vote in local elections but then prevent them from voting in federal elections. Fortunately, the Electoral College eliminates that problem. I really don't care how many non-citizens vote in California elections because California still only gets 54 EV's and I can reasonably assume that they would go to the Democrat even if every last vote was legitimate.
Providing evidence of illegal activity is inherently difficult because the activity is illegal and thus those who engage in it do their best NOT to leave a record. That said, Chicago has LONG been known for illegal voting. The joke in Chicago is that "Grandpa voted Republican his whole life but ever since he died he's been voting Democratic".
It is mostly a Democratic endeavor for simple reasons that explain most crimes, motive and opportunity. Democratic motives are higher than Republican ones because Democrats claim that Republicans are "Literally Hitler" and obviously any moral person would be willing to break minor election laws to stop "Literally Hitler".
Leaving aside the motivation issue, Democrats have a lot more opportunity because nearly all of the Country's big cities are politically controlled by Big City Democratic Political Machines and there is nearly zero opposition in power.
Only recently has this started to be mirrored by an increasing lack of opposition in power in Republican areas outside of the Big Cities. As working class whites have increasingly found themselves abandoned by the Democrats, there just aren't many Democratic voters in Republican areas anymore. Over time this will increase opportunity for Republicans but for now opportunity still favors Democrats.
There is also a volume issue. Even if a Republican elections official in a small suburban County has an easy and untraceable opportunity to cheat for her side, she knows that she simply can't cheat all that much. There are limits numerically to any vote-fraud scheme before it becomes manifestly obvious. In a small Suburban County this means that any cheating done will have only a negligible impact on statewide elections which further erodes the motivation.
The underlying situation is different if you are a Big (Corrupt) Democratic Machine Political operative in a Blue City in a Purple State (Milwaukee, Detroit, Pittsburgh, Philly, etc).