It wasn't. at least not in the way that phrase is meant.
Or to be more precise, there is no evidence that would logically lead to the conclusion that the election outcome was different than what it would have been if every single vote cast was a valid vote correctly counted.
It is a mistake to confuse the fraud which exists, is widespread, and happens in every election, for "proof" that this particular presidential election outcome was changed by deliberate fraudulent action.
Unless you care to, I'll refrain from posting another littany of the common "specific" charges, and their completely non-fraudulent explanations. Most people don't care to see that information, as it doesn't help the conspiracy talk to get confused by facts.
There is plenty of evidence of various types of fraud and cheating that seemed to have been carefully targeted to 'Rat infested cities and counties in swing states. (see http://nachumlist.com/obamafraud.htm) It certainly seems strange that reputable pollsters had Romney with a slim lead nationally in the popular vote, and slightly ahead or even in all the battleground states on the day before the election. So how do you explain the discrepency between these final poll results and the alleged results of the election? Why did these pollsters fail to measure accurately what supposedly occurred in the election? There may have been some other factors that played a role, but the most logical explanation is 'rat fraud and cheating. Simply put, the polls are designed to forecast the outcome based on a model which assigns one and only one vote to each eligible and likely voter. No pollster can handle and forecast alteration in the numbers induced by the fraud and cheating!
Why were the professionals managing Romney's campaign "shellshocked" on election night? Because their internal pollsters, assuming a fair election, had pretty much assured them that they were likely to win. In other words, they were blindsided in such a way that 'Rat fraud and cheating would be by far the most logical way to account for the discrepencies! (Yes, the Romney people lacked the guts to say out loud that such was the case, but read between the lines.)