Ryan Lizza claims then-girlfriend Olivia Nuzzi cheated on him with GOP presidential candidate Mark Sanford in 2020

Disgraced political reporter Olivia Nuzzi — who is set to release a tell-all book on her “sexting” affair with Robert F. Kennedy Jr. — was accused by her ex-fiancé Ryan Lizza Monday of sleeping with former South Carolina Gov. Mark Sanford in 2020 while the pair of journalists were dating.

Lizza claimed he uncovered a secret love letter on hotel stationery that proved his then-live-in girlfriend had been cheating on him with Sanford — infamous for his own extramarital affair — during his 2020 presidential campaign in a Monday newsletter post titled “Part 1: How I Found Out”.

The ex-Politico writer who ended his engagement to Nuzzi after it was revealed she was involved in a “sexting” relationship with Kennedy wrote that the discovery came when his sweetheart returned from what she told him was a reporting trip, leaving her Herschel backpack tipped over beside the bed.

In the pile of notepads and papers, he found a page that stopped him cold, according to the entry in his Telos News newsletter.

“If I swallowed every drop of water from the tower above your house, I would still thirst for you,” she had written in a note dated March 5, 2020, according to Lizza. Their home, he noted, had no water tower.

He wrote that another page named the recipient of the letter as “Mark,” which he said confirmed what he feared: an alleged physical relationship with Sanford, a presidential candidate she had profiled months earlier.

By early 2020, the pair had signed a contract to write a book on the presidential race. Nuzzi was spending “more and more time in South Carolina,” which Lizza believed was for reporting.

Instead, the scorned ex now claims she “secretly followed him on the campaign trail” to pursue Sanford, allegedly sending him explicit photos and messages while telling Lizza she was “dealing with a crisis concerning her sick mother.”

He says she admitted the relationship intensified, ultimately leading to a sexual encounter at Sanford’s home the night she went dark and stopped answering messages, Lizza claimed.

“How could we write a book about the presidential campaign if Olivia had a sexual relationship with one of the candidates?” he wrote.