Klain’s exact timeline is still not set, the sources said, and he is expected to stay in the West Wing for a period of time to help with the transition period for his replacement, one of the people said. The State of the Union address is set for February 7.

Klain has long telegraphed his intent to leave at some point after the midterm elections, noting publicly – and in a more detailed manner privately – the grueling and exhausting nature of the position. But Klain’s deeply ingrained presence in nearly every aspect of the West Wing, along with his decadeslong relationship with Biden, had led some White House officials to urge him to stay in the role for an extended period of time.

Still, officials have been quietly weighing his potential replacements, with names including Steve Ricchetti, the counselor to the president and one of Biden’s closest advisers for years; Agriculture Secretary Tom Vilsack; Labor Secretary Marty Walsh; Jeff Zients, the former Obama administration official who ran Biden’s Covid-19 response operation; and Anita Dunn, the senior adviser who runs the White House communications and messaging operation, among others.

Dunn has said publicly she has no desire to serve in the role and has repeated that sentiment privately, officials said, though a person familiar said that Dunn has expressed interest in being the first female chief of staff.

Klain’s decision to step down is not related to the investigation underway about classified documents found at Biden’s private office and Delaware residence, an official familiar with his plans told CNN, with the decision being made before the special counsel was announced.

The veteran political operative’s tenure was marked by a series of key legislative accomplishments for the White House, as well as his predilection for tweeting his opinions on a variety of topics at all hours of the day. Klain emerged as a central figure within the West Wing and a trusted adviser to Biden on decisions big and small.

Klain’s expected decision marks one of the first high-profile departures from an administration characterized by low turnover over its first two years.

Biden announced that Klain would serve in the top job on November 14, 2020, days after the presidential election.

On Friday, Klain commemorated the second anniversary of Biden’s inauguration with a celebratory staff email, and cake.

“Today – at the halfway point of this term for the President and Vice President – we celebrate not only WHAT was accomplished, but HOW it was accomplished: through teamwork, collaboration, mutual respect, and a lot of hard work. This team did so many historic things, and did them in historic fashion, as part of the most diverse and most talented White House staff ever,” Klain told White House aides in the email obtained by CNN.

Klain pointed to job and economic growth, stabilization of the pandemic, climate action, steps to combat racism and promote equity, and efforts to protect democracy in the US and abroad as examples of their successes.

“I am awestruck at what this team has done and how you have done it. So this afternoon I am treating everyone to cake to celebrate our success and all of your hard work,” he said in the email.

Klain worked with Biden in the early 1990s as chief counsel for the Senate Judiciary Committee while the Delaware Democrat was the panel’s chairman. Two years later, Klain led the team for President Bill Clinton that shepherded the late Supreme Court Justice Ruth Bader Ginsburg’s confirmation.

He later became Vice President Al Gore’s second-term chief of staff, and while he left in 1999 amid feuding between allies of Gore and Clinton, he returned to the Tennessee Democrat’s presidential campaign a year later and became the general counsel for his recount effort ahead of what ultimately became George W. Bush’s 537-vote victory in Florida in 2000, handing Republicans the White House.

Klain then worked as a lobbyist and political adviser and was involved in John Kerry’s 2004 presidential campaign. He reprised his role as the vice president’s chief of staff early in President Barack Obama’s administration, working again for Biden.

He left the White House in 2011 to help lead an investment firm but returned in 2014, when Obama named him as his Ebola response coordinator, a skillset he used in overseeing the administration’s Covid-19 pandemic response as Biden took office.

This story and headline have been updated.

CNN’s Eric Bradner contributed to this report.