In 1913, seven years before serving as the 29th president, Harding wrote to Phillips asking her to destroy their correspondence, as both parties were married and Harding saw the potential scandal as a threat to his political career.
"I have been thinking about all those letters you have," Harding wrote to Phillips.
"I think you [should] have a fire, chuck 'em! Do. You must. If there is one impassioned one that appeals to you, keep it . . . [but] please, chuck the extra pictures, letters and verses. They are too inflammable to keep."
Indeed, had they been discovered there would be no doubt about the nature of Harding's affair.
"It flames like the fire and consumes," Harding wrote when he was 45. "It racks in the tortures of aching hunger, and glows in bliss ineffable -- bliss only you can give."
In another letter, Harding asks if Phillips knows how much she loves him, seemingly confirming that the affair was much more than emotional.
"My Carrie, Beloved and Adored...I do love you so," Harding wrote.
"I wonder if you realize how much -- how faithfully, how gladly...how passionately. Yes you do know the last, you must have felt the proof."
The complete collection, totaling over 900 pages, will be made available to the public on July 29 both in Washington and online. A special program about the collection, free and open to the public, will be held in Washington July 22.
Celebrity Families of 2014 [PHOTOS]