Lots of polling shows that Democrats are less likely to vote if they believe Trump is just going to steal the election. Trump's people have no doubt seen that polling too, which might be motivating Trump to be so honest about stealing the election. Win-win, so to speak.
The more he can get the media and Democratic leaders to talk about his willingness to steal the election, the less likely it becomes that he has to steal it to win. Awfully clever, if intentional.
And this point is not remotely controversial among Democratic strategists who analyze polling and have found that people are more likely to vote if they think their vote will matter and count. Here's one piece of a memo where they talk about how to better frame it (3rd row down)
This yougov model finds that undermining faith in the election does not hurt enthusiasm. What I’m reporting on here is the conclusion of Dem strategists who’ve analyzed the polling and argue the opposite, that if you tell people their vote won’t count, they’re less likely to vote https://twitter.com/gelliottmorris/status/1309174101924294664