As I recall at the time, polls kept showing Clinton easing apart the comparable popularities in her favour through (as I would see it) pretty traditionally proving that she was the better/not-worst candidate, only for something to periodically disrupt the whole process and sharply re-narrow/slightly-reverse the gap. Then it'd ease back into her favour again.
It's entirely possible that someone's got a "Biden's Emails!" equivalent set of flingable-poo ready[1] to scissor the polling lines back across each other at a key moment. Though maybe their opposition has the same idea, this time, or at least now more prepared to use it than the prior encounter when (outwardly at least) they tried to 'aim high'.
If/when we get to the Presidential Debates stage, I expect we'll see more about what to expect. Up 'til now, it still seems to be more at the 'I'm the disruptive outsider' stage (by the disruptive outsider currently sitting/yelling in the very focal point of the 'inside' zone, so needs some contortionism[2] to attempt/believe). And I'm already aghast at how it's going.
Here from not-even-the-sidelines[3], this seems to be a pattern mostly evolved and repeated this entire century, with the possible exception of Obama's years - for which I may have rose-tinted biases. I'm not sure I could say it was so much like that in the prior post-Nixon years and really couldn't say much about pre-Nixon times.
[1] Currently, the background rumble seems to be just a general murmur of Soviet-Style Socialism being in control of him, and suggestions about his having dementia. Both of which seem to me to be projection, and I can't even say the latter is wrong but their man has made his demented ramblings part of his 'appeal' so I don't think they can press on that too much.
[2] Simultaneously claiming that "AOC is trying to make you dependant on the State" and "Pelosi is trying to stop me mailing you your checks", for example.
[3] I'm in a separate stadium across the street, but listening in on your baseball game via a streaming feed while my game of cricket is mostly not looking in danger of dismissals or sixes happening.