It would all depend how decisive a victory the Army of Northern Virginia achieved. If they had a total victory then there is nothing between the ANV and DC. At the very least the boost to northern morale from defeating Lee and the ANV doesn't happen. However when is the victory achieved? if it's on day one does the Union army continue to fight at Gettysburg? Likely this just moves the battle field elsewhere and had an entirely different set of what if variables. Too many variables to have a definitive answer. Now there are a couple of other things one needs to consider. For one the siege of Vicksburg is going on at the same time. Furthermore the Union has already captured New Orleans. Cotton backed bonds to finance the war effort aren't worth anything if you can't supply the Cotton to the European bond holders.Public morale in the north would drop, and the civilians would possibly lose their appetite to keep sending their sons to die in a useless war of aggression.
Ideally a larger cohort of northerners would realize the need to rise up and get rid of the Lincoln regime, which was putting its power over the lives of the northern citizenry.
What the south should've done is not put all their eggs into the basket of conventional warfare, and instead flood Pennsylvania with bands of guerrilla fighters. Blaze a trail of destruction through the Union's heartland-- drive a steak through it's will to keep on fighting an illegitimate imperialistic war.
Whereas Grant and Sherman had no compunctions about laying waste to farms and doing harm to civilians standing in their way, Lee did. "It is well that war is so terrible," he said, "lest we grow too fond of it." As his armies advanced northward and captured farms, he instructed his soldiers that whatever food they took from the farmers, they pay for it. He, not Grant, won the moral advantage recognized by history.
