Oh, I've never heard this talking point before. I didn't realize early voting was frowned upon by the right. How is it more susceptible to voter fraud?
Other than giving the fraudsters more time to operate, it isn't. The problem isn't outright voter fraud, but what I refer to as vote harvesting - rounding up dumbasses who otherwise wouldn't vote and either busing them in or gathering absentee ballots. Again, this happens on Election Day, too, but early voting allows a much larger window.
The Dems love this because liberal voters are by their nature more prone to be lazy/disengaged. The Dems would not favor making it as easy and as early to vote as possible if they didn't see an advantage. They've been perfecting vote harvesting since the old Tammany Hall days. The GOP is catching up but that's not the point - vote harvesting dumbs down the voting pool and undermines Democracy.
And no, that doesn't mean early voting isn't a convenience for many intelligent people - of course it is. So would voting online, and it would be an even worse idea for the same reason.
