Not yet, no. But Clinton didn't look to everyone like a high-caliber opponent to a lot of people until after New Hampshire (although I thought he was high-calbier all along), and it occurs to me that a lot of the electorate were really unsure about Reagan until his debate with Carter right before the general election.
Good point.
Another good point. There could be the danger of a repeat of Nader's dishonest 2000 message that there was no meaningful difference between the two major party candidates.