Polling from March 1984 actually showed Hart to be a strong candidate against Reagan:
Gallup had him leading Reagan by 9 points! (With a margin of error of 5 points). In contrast, Reagan lead Mondale by 5 points.
But given Hart's failure to beat Mondale, and his 1988 disaster, I'll reiterate what I've said earlier: I just don't think he'd beat Reagan. Reagan was an incumbent President running on a strong economy. He could attack Hart's McGovernite past, and the GOP would exploit rumors of Hart's affairs. But I do think that Hart could run a much more narrow race against Reagan than Mondale did.
What do other people think?