Haven't you already done a thread about this?
Anyway, the
White House in 1968 certainly had suspicions about the Nixon campaign going against the US national interest. If Claire Chennault was being used by them
to influence the RVN then laws were being broken.
Yet I don't believe any presidential nominee could expose and run against their opponent on charges like this, not in the modern era. America in the twentieth century had moved beyond the Bloody Shirt. It'd become both more politically sophisticated and less, how shall we say,
robust. Which is why a hardass politician like Johnson was willing to call a halt to the bombing to help Humphrey, but unwilling to pull the trigger on Nixon's alleged crime. One was doable, the other unseemly.
Calling out the Republican campaign would have been too much like the stuff of Depression era conspiratorialists (Long, Butler, Nye) or Charles Lindbergh's attack on foreign influences at the beginning of the war.