A major political party POTUS candidate should be seen as having standing in any well-plead matter when wrongful behavior by any party has potentially denied them, is potentially denying them, or will potentially deny them, the office of POTUS, that they campaigned hard for, that their political party put its imprimatur behind, and for which their individual supporters, as U.S. citizens, cast their vote. But puruant to the U.S. constitution, original jurisdiction at SCOTUS, where the nine Justices there stand ready to serve as a collective trial judge, is not available for individual people to appear as plaintiff or defendant.
Trump may have to wait until after the electors are selected or even after Biden becomes president in order to have standing.
My understanding is that standing only happens after someone is harmed. Trump hasn't yet been harmed because Biden is not yet president.
I remember what happened in New York when Bloomberg passed that law preventing stores from selling large soft drinks. The stores tried to bring the case early because they didn't want to buy a bunch of different sized cups and throw away the large ones in preparation for the law to change, if there was a chance they could win.
But the court said they had to wait until they could show a financial hit.