"Here's the thing you have to understand about Ted Cruz," Franken writes in the book, a excerpt of which was published by Axios.
"I like Ted Cruz more than most of my other colleagues like Ted Cruz. And I hate Ted Cruz."
In another passage, Franken writes that, for Senate Democrats, a "difficult cruise" is Ted Cruz rather than one on Carnival ships, reports The Washington Post.
Even before Trump was elected President, both Republicans and Democrats called Cruz "arrogant" and "rude." Cruz, who ran for president in 2016, is a far-right Republican who opposes same-sex marriage, Obamacare, and immigration reform.
The Minnesota senator and former SNL writer's book comes out on May 30 and contains an entire chapter on Cruz entitled "Sophistry."
A staunch Democrat who supports a number of progressive causes, Franken has frequently challenged Cruz on everything from climate change to the ongoing Russia investigation.
Cruz called Franken "obnoxious" and "insulting" earlier this week for writing about him in his book.