"As a fellow comedian I long admired Al Franken", Colbert went on.
As the storm of allegations has swirled, Trump, who himself has faced more than a dozen allegations of sexual misconduct, had largely avoided comment.
When asked by reporters on Friday the difference between the allegations against Trump and those against Franken, White House spokeswoman Sarah Sanders replied: "Senator Franken has admitted wrongdoing and the president hasn't". "I am asking that an ethics investigation be undertaken, and I will gladly cooperate".
Sanders said that Trump should not be investigated because "in one case specifically, Sen". Franken, D-Minn., who is accused of having forcibly kissed and groped a radio news anchor a decade ago when entertaining USA troops overseas, as "Al Frankenstien".
"She [Clinton] said 'look, I worry about everything from his past because it tells you how he behaves in the present and the future.' What do you say to that as it relates to these allegations against the president?"
"I'm automatically attracted to handsome - I just start kissing them", he said in the audiotape, which surfaced during the campaign. Now he is betting that in this highly partisan environment, he is free to weigh in on controversies involving other men - particularly if they're Democrats like Franken and the now-deposed movie producer Harvey Weinstein, the party donor whose actions spurred the national discussion of sexual harassment.
Among those noting the different reactions: Hillary Clinton, who lost last year's election to Trump.
Jessica Leeds, who alleges that Trump groped her on a flight in the 1980s, said she's "extremely disappointed" that punishment for Trump has stalled. "You can do anything".
Supreme Court judge Clarence Thomas's Senate confirmation hearing in 1991 was overshadowed by allegations he sexually harassed a staffer, Anita Hill, charges which he denied.
After the Franken news broke this week, including a 2006 picture of the future Minnesota senator pretending to fondle a woman who was asleep, Trump took to Twitter.
In his tweets, Trump focused on the photographic evidence against Franken, suggesting that perhaps worse photos exist. "Where do his hands go in pictures 2, 3, 4, 5 & 6 while she sleeps?" read the president's first tweet, sent just after 10 p.m.in Washington.
Many Democrats have come out saying Al Franken should step down and that his behavior is unacceptable as the climate towards sexual harassment, even those that are decades old and from a culture when it was considered normal or flirtatious behavior to grope women without their consent, can cost someone their job.
Ten minutes later, he added: "And to think that just last week he was lecturing anyone who would listen about sexual harassment and respect for women".
President Donald Trump has insisted, for months, that the Republican tax plan he supports won't benefit him.