Credit: Andrew Harrer/Bloomberg via Getty Images | Gregory Bull/AP Photo

Sen. Barbara Boxer, D-Calif., sent an open letter to San Diego Mayor Bob Filner urging him to resign from his post as he faces over a dozen of allegations of sexual harassment.

"Now I must say this directly to you: Bob, you must resign because you have betrayed the trust of the women you have victimized, the San Diegans you represent and the people you have worked with throughout your decades in public life," Boxer said in the letter.

Boxer, who said the revelations about Filner "have shaken me to my core," has publicly called on Filner to step down, but the letter is her first direct appeal to the mayor.

"Bob, you have already hurt so many people. To avoid hurting your victims and the people of San Diego more than you already have, you should step down immediately," Boxer said.

Earlier this week, Filner was disinvited from a September trip to Washington, D.C., with the San Diego Regional Chamber of Commerce.