There are two special elections happening later today. The Georgia one is getting all the attention, since it looks very close, while the South Carolina election is expected to go Republican. Either way the Georgia race goes, it promises to be interesting, not because it will change a great deal in absolute terms, but because everyone's suddenly paying attention.
If Ossof wins, expect the Democrats to take it as a massive victory to rally behind. They'll try to ride it all the way to the midterms, selling it as evidence that the people are fed up with the Republicans generally and Trump in particular. I doubt much will change for the Republicans, even though the pundits will doubtless call it a staggering blow to their pride and a condemnation of their policies and whatnot. They, too, will turn to the midterms as being more important.
If Handel wins, it could actually be more of an upset. Not that I think it's unlikely; I'd still give her at least even money to win. Rather, if the Republican leadership takes it as vindication of their actions, we might actually see something of a split from Trump. It's a long shot, of course; more likely, they'll double down on support for the White House, especially since Trump has publicly endorsed Handel.
By the way, is that an unusual thing, for a sitting President to publicly back a particular candidate in a local election? Maybe it's just the publicity around this particular race, but I don't remember having seen that before. It just seems weird to me.