It's absurd for Trump to be talking about a fixed election. The person who should be complaining about one of those is Bernie Sanders for the Democratic primary.
Oh, Sanders did so. He was the one who first complained that the elections are "rigged."
(Trump constantly quotes Sanders on "rigged," on "crooked," and more to appeal to those Sanders supporters who now support Trump. They reveal themselves to have supported Sanders not for his liberal stances but for other reasons. . . .)
I do not think it is out of propriety for Mr. Trump delay his commitment to accept the election results. He is a businessman, he knows how to apply pressure to make sure he gets a fair and acceptable deal, and he know that there have been numerous changes in voting processes, perhaps not nationally, but certainly at the state and local levels. He is putting pre election pressures on every relevant official, every voting site, and every means of voting currently allowed, nationwide, to scrutinize their operations and more closely supervise their personnel.
Is it the tradition the press and others insist that he continue by their scorning his debate reply on the commitment issue? Or, do they wish to deny him his legal right question the process should he see fit and necessary?
In this election year, I think that he is doing all voters a service by putting pressure on election officials! How can you folks disagree with that?
When was a candidate ever asked such a question in a debate? I can't recall ever.
There was, a few years ago, the interview where Sam Donaldson confronted Mike Dukakis with 'you just don't get it, do you? You're going to get creamed' or something to that effect. It was generally viewed as cruel.
Of course it's hard for Trump to get sympathy for anything, given his demeanor, but that doesn't mean people aren't sandbagging him.
The answer to your question, of course, is when did a candidate, prior to a debate, constantly call for his supporters to go to the polls to "watch" other voters in an already-"rigged" election?