Biden on Thursday announced his slate for new leadership at the Democratic National Committee, including former South Carolina U.S. Senate candidate Jaime Harrison to be the next chair of the DNC.

Harrison, who lost the 2020 race to GOP Sen. Lindsey Graham last year, despite breaking numerous fundraising records, will officially become DNC chair after the full committee votes on Jan. 21, the day after Biden's inauguration.

Graham congratulated Harrison on the news in a tweet earlier Thursday.

In addition to Harrison, Biden also announced the names of four DNC vice chairs, all of whom were prominent backers of his presidential campaign, including Michigan Gov. Gretchen Whitmer, who served as a campaign co-chair.

Whitmer is joined by Atlanta Mayor Keisha Lance Bottoms, Illinois Sen. Tammy Duckworth and Texas Rep. Filemon Vela as new DNC vice chairs.

House Speaker Nancy Pelosi called Harrison an "inspiring, energizing and influential" in a press release.

"As former Chair of the South Carolina Democratic Party and senatorial candidate, Jaime brings an outstanding track record of success -- from galvanizing the grassroots, mobilizing millions and shattering fundraising records -- that will be critical during this pivotal moment for our Party and Country," she added.

-ABC News' John Verhovek and Beatrice Peterson

During an unscheduled stop on Capitol Hill, Vice President Mike Pence greeted National Guardsmen and telling them that he had just come from a briefing where he heard about their efforts.

He thanked the personnel for "stepping forward for your country" to make the historic transfer of power possible.

"It's been my great honor to serve as your vice president and I want to thank you for your service," Pence said.

The vice president then spoke to individual guardsmen asking them where they are from and how long they served.

About 20,000 National Guard members are expected in Washington this week to protect the city ahead of Biden's inauguration.

"Have a safe inauguration," Pence said.

-ABC News' Elizabeth Thomas and John Parkinson

Congressional Democratic leaders commended Biden's announcement outlining his COVID-19 emergency relief proposal.

"The emergency relief framework announced by the incoming Biden-Harris Administration tonight is the right approach. It shows that Democrats will finally have a partner at the White House that understands the need to take swift action to address the needs of struggling communities," House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer said in a statement.

"We will get right to work to turn President-elect Biden's vision into legislation that will pass both chambers and be signed into law," they added.

"The robust package announced by President-elect Biden and Vice President-elect Harris shows we now have the necessary leadership in the Executive Branch to turn this around and we are ready to get to work to deliver aid without delay," Pelosi and Schumer said.

-ABC News' Mariam Khan

Biden will propose a $1.9 trillion, “American Rescue Plan" to address the ongoing COVID-19 vaccination effort and immediate economic need to Americans in evening remarks, transition officials disclosed on a call with reporters Thursday.

According to officials on the call, the rescue policy is only one part of Biden’s overall policy, with a “recovery” phase expected to be introduced in February. They said Biden’s rescue policy will include three components: Tackling vaccine distribution, ramping up testing and getting kids back in school within the first 100 days of his administration; delivering $1,400 per-person direct stimulus checks and expanding unemployment benefits, including $400 weekly benefit through September on top of state benefits; and delivering aid to states and communities to support small businesses, addressing housing insecurity by expanding the eviction and foreclosure moratorium through Sept. 2021.

More than $1 trillion of the $1.9 trillion price tag will be used for direct stimulus, officials said, while $400 billion will go toward COVID-19-related projects, including the nationwide vaccination program, and $440 billion will go toward relief for communities and businesses.

Biden will also call for Congress to provide $350 billion for state local and territorial governments, an effort to keep essential workers on the job.

If enacted, it would be one of the largest and most expensive economic stimulus packages in U.S. history.

-ABC News Molly Nagle, John Verhovek, Beatrice Peterson and Averi Harper

House Democrats have opened an investigation into what law enforcement and the intelligence community knew about threats to the Capitol ahead of the Jan. 6 siege.

The investigation from the House Intelligence, Oversight, Homeland Security and Judiciary committees will also examine whether anyone with security clearances -- current or former National Security, Defense, Justice or Homeland Security officials -- participated in the riot.

The investigation will also examine the federal law enforcement response in the aftermath of the attack.

"The Committees will conduct robust oversight to understand what warning signs may have been missed, determine whether there were systemic failures, and consider how to best address countering domestic violent extremism, including remedying any gaps in legislation or policy," committee members wrote in a letter to the FBI, DHS and Office of the Director of National Intelligence.

"This still-emerging story is one of astounding bravery by some U.S. Capitol Police and other officers; of staggering treachery by violent criminals; and of apparent and high-level failures -- in particular, with respect to intelligence and security preparedness," the letter later stated.

Democrats plan to request documents and briefings from administration officials as part of the investigation -- just one of several looking into the Capitol attack.

-ABC News' Benjamin Siegel