Franken, a Democrat, says he speaks with Republican senators privately, and that some are very concerned about President Donald Trump's actions and even his mental state.
"I haven't heard a lot of good things and I've heard great concern about his temperament," he told Bill Maher on the show Real Time.
Franken also joked that because Republicans now control the US Congress, an impeachment of the new president — a lawsuit has been filed that alleges the president is in violation of the constitution because his businesses receive payments from foreign governments — is "months and months away."
"I know everybody wants a quick fix on this," he said, "but this is going to be a bit of a marathon."
However, the Democrat said "this week has been incredibly encouraging," with hundreds of people showing up for town halls and making their voices heard. "There's a tremendous amount of energy and I want to encourage people to keep that up. It really helps," the Minnesota senator said.
Republicans Jason Chaffetz of Utah and Diane Black of Tennessee were both shouted down at town hall meetings with their constituents, often about concerns the president will get rid of the Affordable Care Act enacted by his predecessor and leave them without health insurance.
"It works, we got hundreds of thousands of calls about Betsy DeVos at the Senate and that makes a difference," Franken said. "She is on notice now, and she's not going to be able to do a lot of the things that I think she wanted to do because of that. What you people do makes a huge difference."
Franken also said the president "has got to stop" saying millions of people voted illegally in the last election, given that there is no actual evidence of his claim. "It's very disturbing," he said.
"This is very, very, very disturbing, that the president has a habit of either imagining things or making stuff up. That's really outside the norm of not just presidents, but of human beings," Franken concluded.