It was the final straw.
The man busted for allegedly trying to extort David Letterman told confidants he saw his live-in lover making out with the married CBS “Late Show” host — and that’s what friends believe sparked his vengeance, sources told The Post.
Robert “Joe” Halderman said he made the shocking discovery as the two were parked at the end of his driveway in Norwalk, Conn., in August on one of the many nights Letterman drove staffer Stephanie Birkitt to the home she shared with the “48 Hours” producer-journalist, the sources said.
Halderman and Birkitt broke up a short time later, and she moved into a Manhattan apartment.
Fueled by rage and jealousy, Halderman allegedly set his sights on Letterman — vowing to make him “miserable” by exposing his infidelity if the funnyman didn’t pay a $2 million blackmail, sources have said.
“You have here someone who [went through] a divorce, losing children, his world is falling apart, and the one thing you can hold onto wasn’t what he thought it was,” said a source familiar with the case.
The bombshell bussing incident, if true, suggests Birkitt and Letterman remained a hot item long after the comic wed longtime girlfriend Regina Lasko.
It also appears to explain why Halderman waited until September to hatch the twisted revenge plot — rather than set it off last December when he first discovered the affair through Birkitt’s salacious diary entries and e-mails to Letterman.
A Letterman spokesman had no immediate comment.
The funnyman was equally mum about the scandal at yesterday’s “Late Show” taping — the first time since the scandal broke that he didn’t joke on-air about his admitted infidelity or the alleged blackmail plot.
Halderman, 51, and Birkitt, 34, lived together for years in the Norwalk house, which he bought after a bitter divorce in 2004.
When he first unearthed her Letterman fling in December, Halderman demanded that Birkitt put an end to it if she wanted to remain with him and she agreed, sources said.
But Birkitt remained an employee of Letterman’s Worldwide Pants Inc., which produces the “Late Show.”
In March, Letterman married Lasko, his girlfriend of 23 years and the mother of his 6-year-old son, Harry.
Also last spring, Halderman’s ex-wife moved with his 11-year-old son from Connecticut to Colorado. The loss of his son’s companionship “devastated” the producer, sources said.
In early September, Halderman allegedly dropped off a package in Letterman’s limo that included the outline of a movie screenplay about a philandering TV talk-show host, as well as salacious excerpts from Birkitt’s diaries.
In subsequent meetings with Letterman’s lawyer, Halderman allegedly demanded $2 million in exchange for not making details of the affair public.
Halderman was arrested last Thursday in Manhattan, hours before Letterman shocked his studio and TV audiences by revealing his past affairs with multiple “Late Show” staffers, and that he had been the victim of an extortion attempt.
Employment lawyer Louis Pechman, who is not connected with the case, said that if any Letterman staffers wanted to sue, they likely would have to limit their claim to him and Worldwide Pants.
“CBS would not be considered their employer, so [CBS is] basically immune from a lawsuit,” Pechman said.
Additional reporting by Rebecca Rosenberg