WILMINGTON, Del. — President-elect Joe Biden unleashed a broadside Tuesday against President Trump's stewardship of national security, accusing his administration of opening the door to a far-reaching cybersecurity attack that has penetrated the networks of key federal agencies and U.S. companies.

“This attack happened on Donald Trump’s watch — when he wasn’t watching,” Biden said during a year-end news conference. He blamed the outgoing president for “failing to prioritize cybersecurity” by thinning out senior cybersecurity positions and for what he called an “irrational downplaying of the seriousness of this attack.”

“The attackers succeeded in catching the federal government off-guard and unprepared,” Biden said, calling on Trump to publicly identify the perpetrator and confirm the conclusions of his own secretary of state, who has said Russian government hackers are to blame.

The drubbing was among Biden’s strongest attacks against Trump on a policy matter since the November election and stood in contrast to the tone of unity the president-elect has tried to project, even as Trump refuses to concede.

Biden has focused criticism on the president’s unwillingness to accept his election loss. Tuesday’s comments, by comparison, amounted to a direct charge that Trump was derelict in carrying out his duty to protect the nation.

At the same time, Biden went out of his way Tuesday to praise Republicans on Capitol Hill, seizing on a $900 billion coronavirus relief package passed Monday by large bipartisan margins. Biden called the legislation a “down payment” on a more ambitious relief package he will propose early next year in hopes Congress would approve it through the kind of dealmaking he was known for during his long Senate career.

Biden said his plan would include more money to help first responders and health-care workers, expand coronavirus testing and provide a new round of stimulus checks to struggling Americans.

The session with reporters provided a glimpse of a soon-to-be president who is likely to handle the press in a far more traditional way than his predecessor. Tuesday’s sober, generally courteous questions and answers marked a notable departure from Trump’s colorful and frequently hostile exchanges with reporters.

In excoriating Trump while praising other Republicans, Biden signaled that he was looking beyond the president and toward the start of his own term, when he will face pressure to produce results and could be helped by even modest cooperation from GOP lawmakers.

Biden predicted that Trump’s departure, along with Americans’ desperate need for help during the pandemic, will ease collaboration.

That does not mean he’s predicting a honeymoon period, he said. “I don’t think it’s a honeymoon at all. I think it’s a nightmare that everybody’s going through, and they all say it’s got to end. It’s not a honeymoon. They’re not doing me a favor,” Biden said. “I think there’s been a dawning here. And look, you have a different team in town. You have a different team in town. I’m not going to villainize the opposition, but I’m going to stand and say this is what we got to do.”

He offered no such conciliatory words when it came to Trump, who continues strategizing on ways to overturn the results of the election. “His failure will land on my doorstep,” Biden said of the cybersecurity breach.

Trump has argued that the danger from the breach is being significantly exaggerated.

“The Cyber Hack is far greater in the Fake News Media than in actuality,” Trump tweeted on Saturday, despite a federal alert in recent days that called the widespread cyberespionage campaign “a grave risk to” government agencies and the private sector.

He added, “I have been fully briefed and everything is well under control.”

On Tuesday, Biden said he has seen “no evidence” that the damage from the hack of federal computer systems is under control, and he noted that Trump “hasn’t even identified who’s responsible yet.”

Top administration officials including Secretary of State Mike Pompeo and Attorney General William P. Barr have cited Russia as the likely culprit, but Trump has shied away from that conclusion, suggesting on Twitter that the true culprit “may be China (it may!).”

Biden dismissed such rhetoric. The president “still has a responsibility as president to defend American interests for the next four weeks,” the president-elect said. “But rest assured that even if he does not take it seriously, I will.”

Trump’s aversion to calling out the Kremlin for its malign activities in cyberspace and his deference to Russian President Vladimir Putin has been a pattern of his presidency.

Biden said planning for the attacks began as early as last year. Several federal agencies, including the Treasury, Commerce, Energy, State and Homeland Security departments and the National Nuclear Security Administration, have said they were targets.

Adding that the cyberattack was “carefully planned and carefully orchestrated,” Biden said that when the culprit is clearly identified, “they can be assured that we will respond, and probably respond in kind. There’s many options, which I will not discuss now.”

Some members of Congress have described Russia’s actions as an “act of war.” But Biden declined to go that far Tuesday, describing the hacking only as “a grave risk.”

Also Tuesday, Biden continued to fill out his White House team, announcing Bruce Reed, a longtime Democratic operative and strategist, as his deputy chief of staff, serving under Ronald A. Klain. Left-leaning activists, concerned about Reed’s centrist approach, had urged Biden not to put Reed on the White House staff.

But in choosing Reed, Biden continued his pattern of naming people he has worked with and trusts. Biden also announced that Gautam Raghavan, a favorite of many on the left, will be deputy director of the Office of Presidential Personnel, a possible effort to soften activists’ disappointment with the Reed pick.

Raghavan has, among other things, been a top aide to Rep. Pramila Jayapal (D-Wash.), chair of the Congressional Progressive Caucus.

Biden was asked at Tuesday’s news conference why he had not yet named his pick for attorney general, one of the highest-profile remaining vacancies on his senior team. Both Barr and Jeff Sessions, Trump’s first attorney general, regularly found themselves under pressure from Trump and facing accusations of politicizing the Justice Department.

“We’re looking for a team who will instill the greatest confidence in the professionals at DOJ to know once again that there is no politics,” Biden said.

He made reference to the unusual nature of his Cabinet search process, especially the close attention being paid to diversity both by the transition team and outside groups.

“As you know, there’s been a great debate about in every single appointment, whether or not people think there are enough African Americans, enough Hispanics, enough Asian Pacific Americans, enough people who are new and young,” Biden said. “So we’re just working through it. It’s not by design. There’s not an obvious choice in my mind.”

Ellen Nakashima contributed to this report.