Donald Trump has never talked Civil War or Armed Rebellion; let's make this clear.
I would suppose that in questionable situations, he'd mound legal challenges, demand recounts, etc. He should if he honestly thinks that something is rotten in Denmark (or wherever) in the vote count. Al Gore did, and he was right to do so. In a perfect world, every ballot in Florida would have been recounted by hand.
I've thought about Trump's statement last night. It's compared to several things. It's compared to Nixon not challenging Kennedy in 1960. Perhaps this is a bit of those days seen from these days, but if Nixon, indeed, was the victim of voter fraud in 1960, he should have pushed the issue then. If voter fraud is a big issue in American politics, there will NEVER be a good time to do something about it because it will only be an issue in close and controversial elections. I think the America of 1960 was adult enough to deal with the issue of some of its political leaders not being saints.
http://www.digitalhistory.uh.edu/disp_textbook.cfm?smtID=2&psid=3109The other thing it's compared to is the election of 1876, where Samuel J. Tilden was flat-out robbed of the Presidency. There was no question about it, and that was a situation where folks really were ready to take up arms over the issue. The solution avoided open rebellion, but ensured Southern autonomy and ushered in the Jim Crow era. Was that good?
There is certainly ample evidence that any Trump defeat at the polls is what the voters have in mind. But a fraudulent election doesn't become honest just because folks choose to not talk about it. If Trump actually has evidence of electoral dishonesty, he shouldn't just take it silently. I wouldn't expect Hillary Clinton to, either.