how I know voter fraud is a serious problem
There has been lots of debate lately over how serious of a problem voter fraud is. Republicans keep pointing out various investigations and convictions for voter fraud, and trying to enforce some minimal integrity by requiring voter IDs and the like. Democrats keep denying that there is any real problem and fight viciously to prevent any legislation that might make voter fraud harder.
That's how I know that voter fraud is an important issue --because Democrats are working so hard to make sure it keeps being easy to do. Does anyone really believe that Democrats would go to the wall for the three or four voters a year who don't have any ID? Unlikely. The most likely explanation for why Democrats are going all-out to keep voter fraud safe and easy is because they believe that they benefit from it by a significant amount.
And they ought to know.