What is a Tricky Dick joke?

The art nouveau Gellert, completed in 1918, was like a beautiful grandmother. Worn, outdated, but with classic style. It’s stained glass and wrought-iron accents were a delight, even overlaid with Soviet-era eyesores, fluorescent lighting, and bulbous sign lettering from the ‘70s.
Feri had snagged us the Richard Nixon suite, so named after the president who laid his head there twice.
“I know. I know. No Tricky Dick jokes,” John said to me.

I know the nickname but what is a Tricky Dick joke?