RE: “I believe RMN (who was at Dealey Plaza that day)...”
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I never heard that!
Source says Nixon was in Dallas on other business on Nov. 21 and left Nov. 22...
After meeting various bottling-plant executives the previous day, RN had spoken to reporters that night, delivering a vigorous critique of the Kennedy White House’s policies in his capacity as Republican elder statesman.
In the morning, RN proceeded to Love Field, and boarded a plane which departed a little more than an hour before Kennedy and Johnson’s arrival.
https://www.nixonfoundation.org/2013/11/richard-nixon-november-22-1963/
As big an intelligence guy as RMN was would not have let history pass him by completely, though he may have known and calculated that his presence wouldn't have helped his résumé, as he needed to look sharp to be the Republican nominee in the future.
Thanks for keeping everyone on our toes!