It probably doesn’t help Trump’s election odds. Only way I could see it maybe helping a little is if they leave the seat open until after the election and use that to drive up turnout among the base. If they fill it now, that will just anger Democrats and drive up their turnout instead. Might be a wash anyway even if they leave it open.
This is exactly what they’ll do. It’s a gamble, but it’s one with a high chance of reward.
If they fill the seat NOW they lose it as an election issue. Or it backfires on them and riles up Biden voters. There’s no third option.
If they wait until after the election, it riles up Trump’s base and they
will turn out. Liberals will turn out too, but there’ll still be a portion of them who’ll be angry about Bernie losing or whatever and not vote for Biden.