it seems unlikely that either general (or any general) could have postponed the fall of Atlanta until after the election
I'm in the opinion that Johnston could have held out longer than he did. He fell back from Rocky Face down pass two main rivers before he even hit the open area south of the Kennesaw line. There he fell back over the last river, Chattahoochee on into Atlanta.
Johnston fell back at times when their wasn't much push against his line. The lines that Johnston fell back from were entrenchments that were prepared even before he retreated back to them. Hard to believe they could even dig them and prepare them as fast as Johnston was moving toward Atlanta. Sherman even had a hard time keeping his supplies up to the front they were moving so fast south.
"IF", Johnston would have defended these areas and manovered his army to the left and right more often he could have defended those well prepared defensive lines a lot longer. The A.O.T. wasn't that spread out to begin with. They were pretty tight and close already, which at times Johnston did move part of his army toward the stress points but soon fell back without a battle.
The only time Johnston defended a place worth mentioning was on the Kennesaw Mountain line and when Sherman did attack him there, he was repulsed. What happened after that repulsed? Just a week later without any more battles, Johnston fell back toward the Smyrna line which he only stayed there long enough to make sure his army crossed the Chattahoochee. Johnston did not even attempt to hold that last river line, he fell on back beyond Peachtree Creek and settled in for Sherman to march close to Atlanta to set up his artillery to bombard the city itself day and night.
Johnston could not see any type of reasons) to attack Sherman at all during this long trek through the mountainous area of Georgia. Johnston did not use his defensive lines to hold back Sherman at all during this time. All Johnston did was react ot what Sherman did and most of what Sherman did was faint toward the Confederate left and Johnston high tailed it out and went to his next prepared defensive line.
Sherman was not about to lose contact with the W&A railroad, that was his life line for his supplies. Johnston knew that but for the life of me, I am no train soldier but knowing the RR was his life line, go ahead and let Sherman slide around , this makes for an advantage to Johnston to hit which ever part of Sherman's army was out forging about Rome, Dallas, Georgia. But just don't pick up and leave an entrench position with you striding the RR yourself. Yes, that RR which Sherman would not and could not afford to get far from during the movements toward Atlanta. ( It was hard enough to sustain only parts of Sherman's army with no army before him when Sherman march to to sea began)
I do believe Johnston could have held out past November if he had decided to fight for every inch. Sherman would have used up a lot of his troops doing this. After the election of 1864 and Lincoln NOT being elected...well that is another "what if" but we do know what happened because Lincoln did get re-elected.
The cool thing is we can analyze this campaign and see the outcome, they could only hope to know what we know now.