I think there's two main reasons this keeps coming up every time a poll shows that Harris has even a chance at winning:
In 2016, Hillary Clinton was way ahead in the polls, only to lose. This was due to many factors, but one that gets a fair amount of blame is that a lot of people hated her and didn't want to hold their nose and vote for her. Staying home was a lot easier when she was supposed to win by a wide margin.
While a lot of people pushing third party candidates were never going to support Harris, there are also those who think that it's ok to vote third party in states that aren't seen as in play in order to "send a message" without risking a Trump win. But if the polls are off and that state is closer than expected, those votes could still cost the election.
Historically, being ahead in the polls helps a candidate. The bandwagon effect is real, and can help drive up turnout and shift how people perceive an election or issue. That's why partisan polls designed to skew the numbers have been around for so long.
Either way, it doesn't hurt to remind people that no matter what they expect the outcome to be, we need as many votes as we can get. Bigger margins can help fight off conspiracy theories and legal challenges, and more turnout in down-ballot races can make the difference between gridlock and real progress.
I'm a bit skeptical about the idea of people staying home in large numbers specifically because it was supposedly safe. But I think you can get to about the same place through an enthusiasm gap.
People who might have been moved by a better candidate and/or campaign but weren't very motivated by Clinton stayed home. It's possible that some might have been swayed if the race was neck and neck since it would have helped drive home the stakes.
Unfortunately, a lot of people are politically disengaged, and a large portion of the population votes on vibes more than reason and policy.
I lived in a solidly blue state in 2016. I voted for Jill Stein (I know, I know, this was before it was widely known that she is a Russian asset and is generally a shitty candidate across the board, and I regret my choice) because I really disliked--and still dislike--Clinton. If people do that in states that are solidly blue, where there's not any significant risk of a red candidate winning, I'm not too worried. If people do that in swing states to 'send a message', then the message we're going to have is that we're fucked.
And, TBH, I'll be fine either way. I can pass as the 'right' kind of person if I have to. I know a lot of people that can't though.
Some think that if a candidate is doing well or poor enough they won't bother to vote because it won't matter. Kind of like when everyone assumes someone else called 911 when there's an emergency.
Do we have any evidence that that has ever happened, ever ever? Based on the exit polls, we didn't see that in 2016 (Democratic turnout was about what we expected it to be). And we know that people really like to play for the winning team, even if the team is already winning.
I am pretty confident I've heard that the opposite is true--that hopeless feeling like the other side is certain to win, and your vote will not change anything, can get people to stay home--but I haven't heard anything compelling suggesting that complacency can get people to sit out an election.
Yeah, do we have any evidence that people have stayed home because they thought everything was settled and there wasn't any need for them to vote?
This seems to be received wisdom, but this will be my sixth general presidential election, and in that entire time I haven't seen any news or studies or polls (or even any anecdotal stories) about it.
Around half of voters are going to vote for Trump. A decent portion of people voting for Harris are swing voters, meaning they at least consider voting for Trump. A significant portion of people are going to vote for Jill Stein, despite the fact that Trump winning is BAD for just about everything she cares about, and host of human rights she doesn't seem to care about.
Irrationally hopeful voters are at the bottom of my shit list.