This is the argument Gordon Rhea makes in Carrying the Flag (page 8):
"General George G. Meade, the federal army's most recent commander, had won his nation's applause by repelling Lee's foray into Northern territory with his victory at Gettysburg in July 1863. But summer, fall, and winter had come and gone, and Meade had accomplished nothing to reap the fruits of his Gettysburg victory. By the spring of 1864, Lee had brought his army back up to strength, and the defensive mind-set of the Union high command had dissipated whatever promise of success Gettysburg had offered the North. A frustrated President Abraham Lincoln could only wonder whether the years of bloodshed had been in vain."
He writes in The Wilderness (page 21):
"According to Lee's returns for April 20, the Army of Northern Virginia had 63,984 soldiers. Additional troops straggled in daily. By May 2, Lee had managed to assemble a fighting force of about 65,000 soldiers. This was considerably more than he had wielded at Antietam, about the number with which he had won his victory at Chancellorsville, and ten to fifteen thousand fewer than he had with him at Gettysburg."
Allen Guelzo gives a mixed assessment in The Last Invasion pages 463-465:
"Many people assumed that the battle of Gettysburg was, as Blake said, 'a decisive triumph,' and given the forces involved, the length of time elapsed, and the casualties afterward, perhaps it should have been. But not in 1863. 'An army of 60,000 or 80,000 men is not to be knocked in pieces by any such battle as we have fought yet,' Andrew Atkinson Humphreys reminded the Joint Committee on the Conduct of the War when he testified before them in March 1864."
"The Army of Northern Virginia's morale, which sank so low in the weeks after Gettysburg, proved how elastic a factor like morale could be, rebounding through the fall and winter of 1863 to the point where Confederates could open a new campaigning season in 1864 'in fine spirits and anxious for a fight.'"
"Writing more than a year later, Richard Henry Dana believed that Gettysburg 'was the turning-point in our history,' not so much for winning a victory as for avoiding a defeat that would have proved the Army of the Potomac's - and the Union's - last defeat. 'Had Lee gained that battle, the Democrats would have risen and stopped the war. With the city of New York and Governor [Horatio] Seymour, and Governor [Joel] Parker in New Jersey, and a majority in Pennsylvania, as they then would have had, they would so have crippled us as to end the contest. That they would have attempted it we at home know.' So even if Gettysburg was less than decisive in strictly military terms, it was decisive enough to restore the sinking morale of the Union, decisive enough to keep at bay the forces which hoped Lincoln could be persuaded to revoke emancipation, decisive enough to make people look back and understand that the Confederacy would never be able to mount a serious invasion again, decisive enough that the momentum of the war would from now on belong solely to the Union, decisive enough that after Gettysburg, the sun never shone for the South again."