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

Biden took aim at Trump in remarks following a briefing on national security and foreign policy, slamming the current administration for failing to fully cooperate with his agency review teams during the transition -- which Biden argued could leave the country vulnerable.

While commending his own agency review teams for their hard work amid a transition that was already made more challenging by COVID-19, Biden pointedly called out Department of Defense and White House Office of Budget and Management political appointees for, he said, not cooperating for a smooth transition.

"And the truth is many of the agencies that are critical to our security have incurred enormous damage. Many of them have been hollowed out in personnel, capacity and in morale," Biden added.

Later in his remarks, Biden stressed the need for cooperation during the transition, noting that it is a matter of national security to ensure there are no gaps that adversaries can seek to exploit when he takes office.

"We need full visibility into the budget planning under way at the Defense Department and other agencies in order to avoid any window of confusion or catch-up that our adversaries may try to exploit," Biden said. "Right now, we just aren't getting all the information that we need from the outgoing administration in key national security areas. It's nothing short, in my view, of irresponsibility."

Biden said other "Day One" challenges discussed in the briefing will draw on the skill sets of the Department of Homeland Security and the Federal Emergency Management Agency as they aim to "clean up the humanitarian disaster that the Trump administration has systematically created on our Southern border."

"We will institute humane and orderly responses. That means rebuilding the capacity we need to safely and quickly process asylum seekers, without creating near-term crises in the midst of this deadly pandemic," Biden said.

As he exited the stage at the Queen Theater in Wilmington, Delaware, Biden was asked if he supports the measure for $2,000 direct payments to Americans for COVID-19 relief, which the House will soon vote on. He responded with one word: "Yes."

-ABC News' Molly Nagle, Sarah Kolinovsky and Beatrice Peterson