Those antigovernment extremists include QAnon, the conspiracy movement, and the Proud Boys, a far-right organization that Canada named as a terrorist group on Wednesday. Supporters of those groups and others were part of the attack on the Capitol building on Jan. 6, which aimed to stop the transfer of power to the Biden administration.

The issue is a difficult one for the intelligence community. By law, the most influential agencies, including the C.I.A. and the National Security Agency, are not allowed to collect information domestically. But Avril D. Haines, the director of national intelligence, has some oversight of the intelligence arms of the F.B.I. and the Department of Homeland Security, which can collect information domestically. Other intelligence agencies look at foreign attempts to influence American groups.

While preliminary work by Ms. Haines’s office is underway, administration officials said that analysis was unlikely to be completed before April. But there appears to be significant interest in moving quickly on the issue in the Senate. At Ms. Haines’s confirmation hearing last month, a number of lawmakers raised the subject of domestic extremist groups. 

The Senate Intelligence Committee will examine both white supremacist groups on the right, and antifascist, or antifa, groups on the left, though Mr. Warner was quick to say that the danger the groups posed was not the same. “I don’t want to make a false equivalency argument here,” he said, “because the vast preponderance of them are on the right.”

Like the intelligence community, Mr. Warner’s panel could face its own jurisdictional challenges as a handful of other House and Senate groups jockey to play a role in studying the aftermath of the Capitol assault and congressional leaders contemplate setting up an independent commission.

For the past four years, the committee has done extensive work on disinformation efforts. Mr. Warner said that experience could guide the panel as it looks at how extremists groups spread propaganda and how foreign powers amplify it.

Unlike most corners of Capitol Hill, and unlike the House Intelligence Committee, Mr. Warner’s panel has managed to operate, for the most part, with bipartisan agreement. All but one senator on the committee backed its five-volume report on Russian interference. Completed last year, the Senate investigation was perhaps the definitive word on Moscow’s interference efforts and found that Russia had disrupted the 2016 election to help Donald J. Trump become president.

Mr. Warner said on Wednesday that the bipartisan record of the committee was important for him to preserve, and that he intended to begin work with closed-door meetings to make the case to other committee members about the threat the groups represent and how they could be exploited by outside powers.

Democrats and Republicans on the committee have expressed interest in examining antigovernment extremist groups, Mr. Warner said. But he acknowledged the political sensitivities after the Capitol attack and Mr. Trump’s support among far-right factions of those groups. Making the case that antigovernment groups are a problem not only in the United States but also in Europe is one way to build consensus on the issue. The committee, Mr. Warner said, will begin its discussions in private sessions so lawmakers can have a candid and less political conversation.

Beyond an investigation of antigovernment extremism and foreign efforts to promote it, Mr. Warner said the committee would work on pushing for new protections for whistle-blowers and making it more difficult to dismiss inspectors general, government officials charged with finding waste, fraud and abuse.

Mr. Trump last year fired Michael K. Atkinson, the inspector general of the intelligence community. It was Mr. Atkinson who investigated the whistle-blower complaint about Mr. Trump’s call with his Ukrainian counterpart in 2019 and ultimately delivered that report to Congress.

At Ms. Haines’s confirmation hearing last month, Mr. Warner began his questioning by describing how his own views on the Chinese government had changed, thoughts he repeated in his interview. He said he was wrong to have believed that China would democratize the more it was brought into the world order.

“I will astonish you and acknowledge that directionally, Trump was right,” Mr. Warner said on Wednesday.

Mr. Warner said he disagreed with John Ratcliffe, Mr. Trump’s final director of national intelligence, who had argued that China was trying to interfere with the election. But Mr. Warner said he believed China had “a very, very sophisticated effort to influence American policy.”

The Senate committee will also look at Chinese technological investments, building on the work members of Congress have done on Beijing’s dominance of 5G, the next generation of mobile phone networks, Mr. Warner said. He said the United States needed to carefully assess its technology compared with China’s on artificial intelligence, facial recognition and quantum computing.

Having a government role in bringing some manufacturing back to the United States from China was an area of bipartisan agreement, Mr. Warner said, mentioning Senator Tom Cotton, Republican of Arkansas and a member of the Intelligence Committee.

“There is a coalition of the willing to take on the challenge of China,” Mr. Warner said. “China has taken the best lessons of British imperialism and American imperialism, and we find them in a kind of authoritarian capitalism model.”