President Joe Biden's Chief of Staff Jeff Zients held calls Monday with White House staff members and appointees across the Biden-Harris administration following his decision to end his bid for reelection Sunday, sources familiar with the matter told ABC News.

Zients told team members to "have the president's back," keep their "heads down" and "get work done," the sources said.

Biden has told Zients there’s more work to do, including “lowering costs for families -- from health care to housing, implementing historic bipartisan legislation, protecting the freedoms of Americans and standing up for democracy," the sources said.

The sources added that Zients also thanked the team and said: "Together as a team -- and under the vice president's leadership, we did everything from bringing the country back from a once-in-a-century pandemic to historic legislation to restoring America's standing in the world."

He closed the calls by telling staff to "run through the tape" and "finish the job," according to the sources.

-ABC News' Selina Wang

House Minority Leader Hakeem Jeffries stopped short of endorsing Kamala Harris as the presidential nominee yet, but said on Monday that he plans to meet with her and Senate Majority Leader Schumer "in short order."

Jeffries praised Harris, saying: "I'm excited for that meeting. Let me say this -- that Vice President Kamala Harris has excited the community. She has excited the House Democratic Caucus. She’s excited the country."

It's not clear when this meeting will occur. "We are waiting on Leader Schumer to get back to town, as well as to get exact confirmation from VP Harris," Jeffries said.

"We owe a debt of gratitude to President Biden for his tremendous leadership, and I'm also thankful for the incredible partnership that he has had for the last three and a half years with Vice President Kamala Harris," Jeffries said.

Jeffries would not say what he and Biden discussed when they recently met privately.

"President Joe Biden is a heroic, patriotic and transformational figure, and he will go down in history as one of the greatest public servants of all time. That much is clear," he said.

-ABC News' Jay O'Brien and Lauren Peller

In the last 24 hours Future Forward, the preferred super PAC that previously backed President Biden's reelection bid and now Harris' election bid, claims it has secured commitments of $150 million from donors who were threatening to withhold their donations after Biden's debate performance, according to a senior aide at Future Forward.

The super PAC had already reserved $250 million in TV and digital advertising earlier this year.

"Future Forward will continue working to make sure Donald Trump is defeated in this election and those dangerous consequences are never felt by the American people," the senior aide told ABC News.

-ABC News' Gabriella Abdul-Hakim, Fritz Farrow and Will McDuffie

President Biden plans to campaign in battleground states "very aggressively" with Vice President Harris as soon as the doctor gives him the COVID "all-clear," according to a senior administration official.

Biden tested positive for COVID last week.

The president will first return to the White House, then will hit the trail in the coming weeks to campaign for and with Harris, the source added.

The president is still expected to deliver remarks later this week and meet with Israeli Prime Minister Benjamin Netanyahu at the White House. The timing of all of this, the source said, is dependent on Biden's COVID recovery. But the source said they are "hopeful" it will be later this week.

The source added that they want to make sure Biden's "voice" is ready to go and that he can clearly deliver his message.

The senior administration official emphasized that Biden is still "going to finish the job" and that his priorities will remain the same. Part of finishing that job, this person added, is making sure Harris gets elected.

Another official said the president is still "full steam ahead" on his legislative priorities.

-ABC News' Selina Wang