There is genuine concern that the republicans will try to steal the coming election. There is plenty of evidence that they have the mindset to do so.
In addition the wholesale lies by the Republicans and their SuperPacs, (go to Politifact.com  to see) there are many avenues available that may be effective. Certainly the efforts at voter suppression in the guise of fighting a non-existent voter fraud is one example.
However, it turns out that the Republican’s worry about election fraud is valid. They, the Republicans, have been doing it for years. The Republican National Committee recommend to several state party, including Colorado, to hire a canvasing firm that, as it turns out has a long history of election fraud. Note that the Romney campaign is also tied to this company.
Another tactic will be voter intimidation. True the Vote, a Tea Party based organization, held a summit in Colorado. Here is a link to a reporter who attended the meeting and his reactions.
There are also other concerns about election fraud. There have been several instances where the final tally from election machines did not agree with the exit polling. The Carter Center would call that an invalid election. There is great concern about the manipulation of the process as outlined in this article from OpEd News:
and this report from The Free Press
Finally, the Koch Brothers are spending millions of dollars on a gigantic get-out-the-vote effort.
It is obvious that the Republican Party and their billionaire friends will do everything they can, legally and illegally, to defeat the president and Democratic Candidates.