The question is whether anyone should really care. Yes, if you wanted to, you could risk five years in prison and a $10,000 fine to vote for someone else, but we're not sure why you would, since a single vote, or even a few votes, will never make a difference. (Okay, almost never.) Could a group of hundreds or thousands of fraudsters be mobilized to go around to different polling stations on election day and vote for one particular candidate or issue, possibly altering the outcome of an election? It would be difficult to organize surreptitiously, but sure, it's probably doable. But it has never happened.
According to the New Yorker,
"it shows just how limited O’Keefe’s talents are, and how un-ambitious is the vision espoused by the right’s new investigative journalists and those who publish them."
According to the Post:
Under D.C. law, persons found guilty of making “any false representations” as to their eligibility to vote are subject to fines up to $10,000 and up to five years in prison. If the fraud is committed during an election where the presidency or congressional offices are on the ballot, it can also be prosecuted under federal law, which contains the same penalties.