Biden and Vice President-elect Kamala Harris are participating in a virtual briefing with expert members of their national security and foreign policy agency review teams from the Queen Theater in Wilmington, Delaware, and taking the opportunity to draw stark contrasts between Trump's "America First" policy and their incoming White House.

At the top of the briefing, Biden praised the team for their work despite delays in the transition prompted by the Trump administration refusing to recognize Biden's ascertainment as the president-elect for more than two weeks.

"You’ve done it all under incredibly difficult circumstances, dealing with the COVID protocols, delays, delays in the ascertainments, and in a few cases obstruction from the current leadership," Biden said, thanking the group before taking aim at Trump's "go-it-alone approach" of the past four years.

“The truth is, the challenges we face today can't be solved by any one country acting alone. They demand American leadership. They demand cooperation with our allies and our partners. So I’m looking forward to hearing your assessments about the greatest challenges we may inherit and as we restore the principle of American leadership on the global stage -- which we have to and will do," Biden added.

After the briefing, Biden is slated to deliver afternoon remarks on the group's findings.

-ABC News' Molly Nagle

Dr. Celine Gounder, a member of Biden’s COVID-19 task force, said Monday morning there is "no question" the country will see a post-holiday surge of coronavirus cases and, to help offset the worsening pandemic, confirmed Biden will invoke the Defense Production Act once in office to increase vaccine supply.

Gounder criticized the Trump administration for moving too slowly on vaccine distribution and said Biden will force manufacturers to step up by invoking the Defense Production Act, which gives the president the power to direct civilian businesses to help meet orders for products necessary for national defense.

“In the last two weeks, we basically averaged about a million doses of vaccine administered per week. We need to be getting to a million doses per day if we're going to reach the president-elect's target of 100 million doses in 100 days," Gounder said. She went on to explain that invoking the act is "to make sure that the personal protective equipment, the test capacity and the raw materials for the vaccines are produced in adequate supply so that those aren't limiting steps in all of this.”

Gounder also claimed the country will see "a major increase" in testing as well as in genetic surveillance to track new variants of the virus with the incoming administration.

-ABC News’ Brian Hartman

Trump is slated to return to Georgia in one week for a campaign rally on the eve of the Jan. 5 runoff races that are set to determine which party holds power in the U.S. Senate.

Sitting GOP Sens. Kelly Loeffler and David Perdue are facing off against Democratic contenders Rev. Raphael Warnock and Jon Ossoff, respectively, as early voting continues across the state Monday.

Earlier this month Trump stumped for Loeffler and Perdue at his first rally since the November election. There, he assaulted the integrity of Georgia's vote but encouraged GOP turnout in the wake of Biden becoming the first Democratic presidential candidate to win the state since 1992. Democrats are now hoping to cement its status as a swing state.

Biden, too, has stopped in Georgia since the presidential election to campaign for Warnock and Ossoff as the promise of passing the big ticket legislation he’s hoping for hangs on their races. If Democrats pick up both seats, Vice President-elect Kamala Harris will have the power to settle tie-breaker votes in Congress' upper chamber.

The Biden-Harris transition team has announced 12 new members of their incoming staff for the Office of Digital Strategy -- which the transition says will be a “robust” team in the Biden White House.

All 12 staffers announced Monday morning previously worked on the Biden-Harris campaign, which relied heavily on digital engagement amid the COVID-19 pandemic.

"With much of our lives online, it is critical for this administration’s digital efforts to be inclusive and extensive,” said Vice President-elect Kamala Harris in a joint statement announcing the picks.

-ABC News Molly Nagle

Following a briefing with his COVID-19 advisory team on Tuesday, Biden is expected to continue his criticism of the Trump administration in afternoon remarks on the coronavirus pandemic from Wilmington, Delaware, and home in on what his team has characterized as the lagging pace of vaccine distribution.

According to a transition official, Biden will provide an update on the current state of the pandemic and give a dire prediction that, tragically, infection and death rates will rise in the coming weeks. Biden is also expected to address the current administration falling short on its pace for vaccinations, the official said, as well as his plan to get 100 million vaccinations in 100 days.

Biden on Monday slammed the Trump administration for, he said, leaving security agencies "hollowed out" and for creating "roadblocks" for his transition team seeking access to key national security information -- a comment that acting Defense Secretary Christopher Miller has pushed back on, saying the Biden team has already been given more interviews and documents than they initially requested.

Vice President-elect Kamala Harris and her husband, Doug Emhoff, on Tuesday morning are slated to receive their first doses of the COVID-19 vaccine in Washington, D.C., where Harris may also comment on the incoming administration's vaccine distribution plans.

Trump, meanwhile, continued to contest the election on Twitter Tuesday morning -- with three weeks until Biden's inauguration -- criticizing Republican leadership as "pathetic," saying they "only want the path of least resistance" and "only know how to lose." He also attacked Republican lawmakers over their handling of the defensing funding bill as the Senate is expected to override Trump's presidential veto for the first time in his presidency by the end of the week.

After Trump changed course on Sunday and signed the coronavirus relief and government funding bill to avert a shutdown -- though not in time to prevent unemployment benefits to millions of Americans from lapsing -- the House of Representatives on Monday approved Trump's call to increase the coronavirus relief payments from $600 to $2,000.

But its fate in the Senate is uncertain. Senate Majority Leader McConnell has declined to reveal how he will proceed or even whether he’ll bring the proposal up for a vote. Democrats are seizing the opportunity to force Republicans into a difficult position -- either backing or defying Trump.

-ABC News' Molly Nagle and Ben Gittleson