WI: LBJ killed by Secret Service on Nov. 22 1963?

As we all know, JFK was assassinated on November 22nd, 1963. And, according to the book The Kennedy Detail, a Secret Service member nearly shot Lyndon B. Johnson the very night Kennedy died. The Secret Serviceman was guarding LBJ's home when he apparently heard footsteps approaching. The Secret Serviceman cocked his gun loudly as in an attempt to get whomever was approaching to leave. When this did not happen, he raised his weapon and prepared to fire only to find he was aiming at LBJ. So, my question is: had the Secret Serviceman fired and killed LBJ, what would've been the effect? As far as I can tell the Presidency would've passed to Speaker of the House John W. McCormack. Also, the impact on conspiracy theories would probably be indescribable.
 
John McCormick, Speaker of the House, is president. He was 71, so I don't think he runs in 1964. He was expirencd as a congressional leader. I think he could have gotten the 1964 Civil Rights Act passed. I think Humphrey wins the Democratic nomination. He beats Goldwatet in a landslide. The Humphrey Adminstration 1965-1973 is remembered for progress in civil rights and the Great Society or whatever his PR people would have called it. The is no Vietnam War.
 
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