Campbell was popular at a time when most deep music fans were into rock, jazz, or folk, and country was viewed as the music of rural, uneducated, red-necks. That statement wasn't entirely true but at that time, within my sphere of influence, perception was definitely reality.
As it turns out Campbell was much more than a country singer. I learned this a long time ago but only after his initial popularity had faded. As part of the famous 1960's Wrecking Crew he was a highly sought after studio musician. On records he accompanied The Mamas & Papas, Jan & Dean, Frank Sinatra, Elvis Presley, The Monkees, and many more.
Campbell also played on The Beach Boys' Pet Sounds and he was the first person Brian Wilson hired to replace himself on the road. However, he didn't stay long because of his desire to pursue a solo career and the band replaced him with Bruce Johnston.
The Arkansas native was considered a virtuoso on his instrument. Nothing proves that more than this video of Campbell performing "The William Tell Overture" on an acoustic guitar live with an orchestra. I have no idea if this is the version that impressed me so much when I first saw it on TV all those years ago. However, thanks to the digital age you can see it now. If you need proof that this dude was a really fine axeman you'll find it here.