Top Democrats in Washington cried foul after Kevin McCarthy, the new Republican House speaker, released more than 40,000 hours of surveillance footage from the January 6 US Capitol attack to Tucker Carlson, the far-right Fox News host who has consistently downplayed the deadly riot.

The Senate majority leader, Chuck Schumer, told colleagues McCarthy’s decision “poses grave security risks” and “needlessly expos[es] the Capitol complex to one of the worst … risks since 9/11”.

But McCarthy told the New York Times he had “promised” to release the footage, apparently as part of dealmaking with which he clinched the speakership after far-right rebels forced him through 15 nominating votes.

“I was asked in the press about these tapes,” McCarthy added, “and I said they do belong to the American public. I think sunshine lets everybody make their own judgment.”

McCarthy said he wanted to give Carlson “exclusive” access to the footage, but could release it to other outlets later.

Carlson, a prominent voice in far-right media, has claimed the insurrection was a “false flag” attack and generally tried to downplay it without offering evidence. He told the Times he was taking the footage released by McCarthy “very seriously” and had a large team reviewing it.

Nine deaths, including law enforcement suicides, have been linked to the attack on Congress by Trump supporters seeking to block certification of Joe Biden’s 2020 election win, fueled by Trump’s lie about widespread electoral fraud.

Trump was impeached for inciting the attack but acquitted when enough Senate Republicans stayed loyal. He continues to run for the Republican presidential nomination in 2024. The US Department of Justice is investigating January 6 but has not yet acted on criminal referrals regarding Trump made last year by a House committee.

A possible Republican challenger to Trump, his former vice-president, Mike Pence, is expected to fight a grand jury subpoena as part of the justice department’s January 6 investigation.

Pence would be a key witness, offering unique insight into conversations with Trump and the efforts to stop certification of the 2020 presidential election, a process over which Pence ultimately presided.

Pence was at a December 2021 meeting at the White House with Republican lawmakers who discussed objections to Biden’s win. Pence also spoke to Trump one-on-one on 6 January, when Trump was imploring him to unlawfully reject electoral college votes for Biden at the joint session of Congress.

Those two interactions are of particular investigative interest to the justice department-appointed special counsel, Jack Smith, as his office examines whether Trump sought to unlawfully obstruct certification and defrauded the US by seeking to overturn the 2020 election.

However, experts in constitutional law this week told the Guardian that Pence had a good chance of success in his attempt to avoid having to testify by citing the speech or debate clause, the constitutional provision that protects congressional officials from legal proceedings related to their work.

On Wednesday, Hakeem Jeffries, the Democratic House minority leader, followed Schumer in protesting McCarthy’s decision to release January 6 footage to Carlson and Fox News.

“The apparent transfer of video footage represents an egregious security breach that endangers the hardworking women and men of the United States Capitol police, who valiantly defended our democracy with their lives at risk on that fateful day,” the New York congressman said.

Jeffries noted that the House January 6 committee, a panel consisting of seven Democrats and two anti-Trump Republicans which operated in the last Congress but disbanded when Republicans took control of the chamber, had enjoyed access to the footage McCarthy has now released.

The January 6 committee, Jeffries said, was “able to diligently review [the footage] … with numerous protocols in place to protect the safety of the members, police officers and staff who were targeted during the violent insurrection.

“There is no indication that these same precautionary measures have been taken in connection with the transmission [to Carlson] of the video footage at issue.

“Unfortunately, the apparent disclosure of sensitive video material is yet another example of the grave threat to the security of the American people represented by the extreme Maga Republican majority” – a reference to Trump’s campaign slogan “Make America great again”.

In his letter to colleagues, Schumer said the footage showed where cameras are located in the Capitol and other details of security arrangements.

The New York senator added: “Giving someone as disingenuous as Tucker Carlson exclusive access to this type of sensitive information is a grave mistake by Speaker McCarthy that will only embolden supporters of the big lie [about voter fraud and the 2020 election] and weaken faith in our democracy.”