A chorus of Democrats on Tuesday called for Sen. Bob Menendez (D-N.J.) to step down after he was indicted on federal bribery and corruption charges.

Sen. Cory Booker, the other New Jersey Democratic senator, was among the growing number of Menendez colleagues saying it was time for him to resign. A total of 18 Democratic senators by early afternoon had said Menendez should step down.

Booker in a lengthy statement said Menendez is making a “mistake” by not stepping down in order to focus on his legal defense against what he called “shocking allegations” that were laid out by the U.S. attorney’s office for the Southern District of New York on Friday. 

The former Newark mayor said Tuesday that he found the charges facing Menendez “hard to reconcile with the person I know,” but argued that the senior New Jersey senator needs to resign as lawmakers operate at a “higher standard” and that he can no longer operate effectively with his Senate colleagues, with nearly a dozen having called for him to step down. 

“Stepping down is not an admission of guilt but an acknowledgment that holding public office often demands tremendous sacrifices at great personal cost. Senator Menendez has made these sacrifices in the past to serve. And in this case he must do so again,” he continued. “I believe stepping down is best for those Senator Menendez has spent his life serving.”

The move is an extraordinary step for Booker, who has been a key ally of Menendez’s to the point where he defended him vociferously after his 2015 indictment on corruption charges. Those charges were ultimately dropped. 

His call also came amid a tidal wave of resignation calls from within the Senate Democratic Caucus. 

On Tuesday morning alone, 10 Senate Democrats said that the three-term senator should step down, including a number of prominent members: Tammy Baldwin (Wis.), Bob Casey (Pa.), Jon Tester (Mont.), Jacky Rosen (Nev.), Elizabeth Warren (Mass.), Martin Heinrich (N.M.), Mark Kelly (Ariz.), Michael Bennet (Colo.), Amy Klobuchar (Minn.), and Booker. 

Prior to that, Sen. John Fetterman (Pa.) on Friday became the first Democrat to say Menendez should step down, followed by Sens. Sherrod Brown (Ohio) and Peter Welch (Vt.) on Monday.

Calls came later on Tuesday from Sens. Edward Markey (D-Mass.), Maggie Hassan (D-N.H.), Mazie Hirono (D-Hawaii), Kristen Gillibrand (D-N.Y.) and Raphael Warnock (D-Ga.).

Booker had also been the lone key holdout among top New Jersey Democrats. Gov. Phil Murphy (D), along with key House Democrats and Democratic leaders in Trenton, have already called on Menendez to resign in recent days.

However, Menendez has shown no signs of bowing to pressure, saying at a press event Monday that those calling for him to step aside are wrong and doing so out of “political expediency,” and that he will be exonerated when all is said and done.

“The allegations leveled against me are just that: Allegations,” Menendez told supporters and media in Union City, N.J. “I recognize that this will be the biggest fight yet. But as I have stated through this whole process, I firmly believe that when all of the facts are presented, not only will I be exonerated, but I will still be New Jersey’s senior senator.”

There is still one group of Senate Democrats that has yet to side with those calling for his ouster: leadership. Senate Majority Leader Chuck Schumer (D-N.Y.) said Friday that he supports Menendez’s decision to step down from his chairmanship of the Senate Foreign Relations Committee, just as he did following the 2015 indictment, but he declined to go any further.

Sens. Dick Durbin (D-Ill.) and Debbie Stabenow (D-Mich.), the party’s No. 2 and No. 3 Senate leaders, also have declined to call for him to step down. 

Adding to the issues for Democrats, Menendez is up for reelection next year, and his indictment will likely be hanging over the party if he runs for a fourth term. Rep. Andy Kim (D-N.J.) has already announced a primary challenge.

Updated at 2:12 p.m.