It will take an exceptional GOP candidate to overcome the now built in level of cheating.
Republicans gained over 700 elected positions in 2010 from top to bottom, won 63 House seats, have a majority of governors. Plenty of analysts have studied the 2012 voting results and seem to believe Romney lost because conservatives stayed home. If vote fraud was deep and wide, republicans would not be winning all these other races.
I think it's clear some close races are affected, namely Al Franken's senate race and there was a Washington state governor's race about 10 years ago that was clearly stolen by the democrats. Somehow, Scott Walker has been able to persevere in a state that appears easy for fraud.
I tend to think vote fraud is more targeted to specific races, but there is no doubt it goes on and is why dems don't want voter ID. I do agree, Republicans need more than 50% to win to offset fraud that no one can really estimate.