Hans von Spakovsky, a former Republican member of the Federal Election Commission, has this article on the Heritage Foundation’s web page, about fraudulent ballot access petitions in the Indiana presidential primary in 2008. Both candidates who petitioned in the Democratic race qualified for the ballot, and the fraud was not discovered until years after that primary.
Von Spakovsky implies that only the Barack Obama petitions contained forgeries, but actually, both petitioning candidates in that Democratic primary submitted petitions that contained forgeries. Hillary Clinton was the only other candidate who petitioned. Indiana requires 4,500 signatures for anyone to get on a presidential primary ballot.
The article then mentions the 2012 Republican presidential primary in Virginia, when four petitions failed to gather enough signatures, and the Republican primary only included Mitt Romney and Ron Paul
Von Spakovsky concludes that because petition fraud exists, therefore voter fraud in general exists. Von Spakovsky is an ardent support of government photo-ID for voters at the polls. But the point that most people would make, concerning the Indiana and Virginia petition failures and problems, is that the presidential primary ballot access requirements in both states are too difficult. If even candidates such as Newt Gingrich, Rick Perry, Barack Obama, and Hillary Clinton have trouble with such petitions, the ballot access laws should be relaxed. And, in fact, in 2013 the Virginia legislature did cut the number of signatures in half for presidential primary petitions, from 10,000 to 5,000 signatures.