"I have stood for due process throughout my years as a prosecutor and in chairing the Judiciary Committee. I regret not doing that this time. The Ethics Committee should have been allowed to investigate and make its recommendation," Leahy said in a statement.
On December 6, Leahy joined a wide number of other Democrats in urging Franken to step aside after another woman came forward with allegations of an unwanted sexual advance from before he was senator.
"I am concerned that even a prompt Ethics Committee investigation and recommendations will not come soon enough," Leahy said at the time. "He has to step aside."
Politico first reported Monday morning that Leahy privately told Franken of his regret.
Manchin, a West Virginia Democrat who did not previously call for Franken to step down, told CNN's "New Day" that Franken should have remained in the Senate and proceeded through an ethics review.
"I definitely think he should not resign. I think he should submit himself, which he has willingly done and offered to do and go through this complete process of an extensive ethics review," Manchin said, adding that Franken should pledge to abide by whatever the committee recommends.
"And whatever the outcome is, I can live with that. I say, Al, I'm sorry, you have faults that you've never recognized and these ladies have brought this forward. And these ladies should be protected."
Over the last two months, Franken has been accused
of groping and forcibly kissing women in incidents as early as 2006.
CNN has reached out to all Democratic senators who previously called for Franken's resignation. So far Leahy is the only one to publicly express regret. Spokespeople for multiple others -- including Sens. Cory Booker, Sherrod Brown, Patty Murray, Michael Bennet, Debbie Stabenow and Bob Casey -- say the senators stand by their original comments urging Franken to step aside.
Senate Minority Leader Chuck Schumer has not changed his mind on whether Franken should resign, an aide told CNN.
"Schumer and the vast majority of the caucus like Sen. Franken and will miss him, but did what they felt was best and stand by it," the aide said.
New York Democratic Sen. Kirsten Gillibrand is also standing by her call for Franken to resign over the allegations a senior aide told CNN. Gillibrand, who initially supported an Ethics Committee investigation, responded to questions about Franken, saying, "He was entitled to a process, but he was not entitled to my silence," according to the senior aide.
in the Senate was named as Minnesota Lt. Gov. Tina Smith last week. Smith said she expects to move to Washington in January.
A top Minnesota Democrat told CNN that Smith is in Washington on Monday and fellow Minnesota Sen. Amy Klobuchar is hosting something for her and Franken staff later on the Hill.
This Democrat, who is close to Gov. Mark Dayton's office, says there is no sense that anything has changed -- despite what Manchin and others have reportedly said.
"This would be hard to un-do," a Democrat close to Dayton told CNN.
And while his replacement has been set, the Franken's office told CNN last week he does not yet have a departure date -- and is still finalizing his plans.
His office told CNN his resignation date was still being "finalized" on Wednesday. And when pressed by CNN four times about his resignation on Thursday, Franken declined to answer questions on why he hadn't set his date.
An aide to Franken said that the senator is tying up loose ends and working on a smooth and speedy transition, and while no date is certain for his official resignation, they expect his replacement to be in place in early January.
A former senior Senate Democratic leadership aide told CNN Thursday Franken staying put is "unusual" and "odd" -- likely driven by Franken himself.
"I am chalking this up to him being stubborn enough to want to go out under his own set of rules," the aide said.