I believe the only way Trump doesn’t fill the seat is if 4 Republicans defect and refuse to confirm whoever he nominates, and then for Trump to subsequently lose the election. Although, IMO - Most likely any R senator would be crazy to vote against then nominee. It would be a guaranteed way to end their political career.
I expect that Trump will nominate someone shortly, but I think the bolder move for him would be to announce, “I will nominate a conservative constitutionalist replacement for Justice Ginsburg when I am reelected in November.” Once he nominates a justice, he loses a lot of leverage. McConnell will get his pick through, and Republicans will have locked in a conservative Supreme Court for a generation. At that point, voting for Trump loses some urgency for Republicans.
But if he essentially holds the nomination hostage to his own reelection, Republicans will have to go balls out to secure it – even more so than they are now. It would piss off Republican leaders to no end, but what does Trump care? It’d be like the scene from Goodfellas – “You want this Supreme Court seat? Fuck you, pay me.”
The Merrick Garland nomination showed that arguments about Senate process and precedents have zero resonance with voters. What will resonate with voters is showing that the nominee creates an all-but-guaranteed majority to overturn Roe, upend environmental protections, and reign in civil rights protections.