The first time I noticed it was in 1960. That was when I was first old enough to remotely care.
It probably began long before then.
I don’t think the slander of modern american conservative movement is much older than that since it is a response to modern american liberalism. It cant be older than marxism, which dates to the 1880s. It cant be older than leninism, which dates to the early 1900s. It can’t be older than the communists revolutions in eastern europe of circa 1920. I would guess the modern american conservative moviement was born during the great depression when communism movements started sprouting up all over the world. That would put it in the 1930s. The open slander of the american conservative movement in the media would start some time after that since it would take time for the socialists/communists to infiltrate the media. Seems like some time AFTER WWII would be a good guess since during the war the media would be censored.