Once the grafting of Left+Democrats was complete, as it was in 1972 or so, the Soviets could just sit back and watch as the various Democrat machines and unions continued to support "their" party. Why run the risk of exposure?
Useful idiots like Kerry, Clinton, and others had already been "planted".
One day, this anomaly will end. Or, it will if the damage to our education system has not gotten too deep. Sometimes, I'm not so sure. Leftist programs now support a wide range of Americans with tax dollars, which those so supported will ever vote to defend.
Prior to 1965, a candidate like Kerry wouldn't have gotten two votes to rub together. Ten years later, he was a successful politician. Clinton was a smooth-talking womanizer and draft-dodger in 1970. In 1980, he was attorney general of Arkansas, and on his way to the governor's office and thence to the White House.
The wild times of the late '60s enabled that radical shift in American politics.
We always had a Left. However, they were considered nothing more than fringe at best. The '60s, with a little help, created the illusion that they were mainstream, and gave their ideas credibility that they did not deserve.
David Horowtiz's books also give added weight to what you've written.