I highly doubt it's going to be 'on the line' by the time of the election. Trump said he's going to pick a woman and it's probably going to be Amy Coney Barrett (three names, always an indicator of sociopathy). Pretty much a female Kavanaugh without the alcoholism. Catholic anti-choice, hate the poor, will rubber stamp Republicans corruption. Could be on the court for 40 years.
The Dems have no way of stopping an appointment. They'd have to convince a handful of Republicans to switch sides. My guess is a couple of them get a 'free pass' to vote no, or to abstain, if it's politically expedient for re-elections, but it will be stage managed to ensure the vote still passes.
I think it helps Trump slightly
Given Covid and Trump's personality, some nominal Republican voters may have stayed at home
But now with a Supreme Court pick on the line, they may now turn up as they would believe Biden would pick someone very liberal (sigh, I wish)
"I don't like remarks about the troops but a Supreme Court pick is a Supreme Court pick"