So the race is essentially tied at the national level which, if accurate, means that Trump is probably slightly ahead in the electoral college. The debate tonight can move the needle. I believe it has the power to move the needle more for Harris than Trump. Trump is better known than Harris. People either will not vote for him based on what they know, will not vote for him based on what they know or are not certain if they will vote for him. Those not certain know of Trump and in their minds his negatives do not outweigh his positives so much that they have decided not to vote for him yet. Nor in their mind do his positives outweigh his negatives that they have decided to vote for him.
But Harris is a relative unknown. Those not certain about who to vote for or whether to vote at all will want more information about Harris to compare what they know about Trump to make those decisions. Nobody will learn much new about Trump tonight. I mean if he acts in a rude sexist way to Harris will anybody be shocked. But Harris has had so few unscripted moments so far and has stayed out of the light except for teleprompter speeches, this is an opportunity for her to shine, or to crash.
It may be that the debate tonight does not move anything. But if it does it will be because people have changed attitudes about Harris. Thus in comparison to Trump the undecideds will compare her either more or less favorably to Trump.