Donald Trump said the investigation into Russian collusion during the 2016 election “could be one of the biggest political scandals in history” as he dubbed the investigation into election inference “Spygate”.

The White House hasannounced a Thursday meeting between top law enforcement officials and two Republican members of Congress – Trey Gowdy of South Carolina and Devin Nunes of California – to examine classified documents about a government informant’s contacts with Trump’s campaign.

Gowdy is the chair of the House government oversight and reform committee and Nunes, a former Trump transition adviser, leads the House intelligence committee.

And in a turnaround late on Wednesday it was announced that there will now be a second classified briefing on Thursday that will involve a bipartisan group of top lawmakers. Originally, no Democrats were going to attend any meetings or be given access to the sensitive documents, sparking outrage from party leaders.

Trump renewed his offensive against the investigation as a “witch-hunt” by the “criminal deep state”. His statements came only days after he demanded an investigation into any government surveillance of his campaign – something the justice department later agreed to.

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Trump has claimed, without evidence, that the FBI had planted a spy within his campaign. In remarks to reporters outside the White House, Trump insisted that the documents to be shown to Gowdy and Nunes would substantiate his claims. “People are going to see a lot of bad things happened,” Trump said in Washington as he briefly spoke to reporters before traveling to New York.

“I hope it is not so,” added Trump, “because if it is there has never been anything like it in the history of our country.”

Senate Democratic leader Chuck Schumer had said the very fact of the meeting was outrageous, but added that the “only thing more outrageous … is the fact that it’s now partisan.” Other senior Democrats had called for it to be cancelled.

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Late on Wednesday it appeared that the White House and the Justice Department relented.

House and Senate lawmakers from both parties are now set to meet with top intelligence officials on Thursday as Trump continues loudly to air suspicions about the federal investigation into his 2016 campaign.

In recent days, he has been zeroing in on, and at times embellishing, reports that a longtime US government informant approached members of his campaign during the 2016 presidential election in a possible bid to glean intelligence on Russian efforts to sway the election.

The Justice Department said it would host a second classified briefing on Thursday and invite the bipartisan “Gang of Eight” – a group that includes the top Republicans and Democrats in each chamber and the top Republicans and Democrats on the House and Senate intelligence committees.

There were two other late additions to the list: White House chief of staff John Kelly and Rod Rosenstein, the deputy attorney general. The White House press secretary, Sarah Sanders, had originally said that no one from the White House would attend the briefing and that no Democrats had been invited because they had not requested access to classified documents – a remark which further incensed the furious Democrats.

Rosenstein will replace another Justice Department official who was originally scheduled to attend. Trump on Tuesday declined to say whether he had confidence in Rosenstein. Rosenstein appointed special counsel Robert Mueller, who is leading the Russia investigation, and is frequently criticized by Trump.

The meeting comes as new developments in the Russia investigation pose additional threats to the White House. In a court filing, special counsel Robert Mueller asked a federal judge to start the sentencing process for the former Trump campaign aide George Papadopoulos, who pleaded guilty in October 2017 of lying to federal investigators. Papadopoulos, who Trump has attacked on Twitter as “a low level volunteer”, had contacts with a Russia-linked professor about “dirt” on Hillary Clinton and later bragged about that information to an Australian diplomat.

Trump’s embattled former attorney Michael Cohen also faced additional scrutiny when the BBC reported that he was paid $400,000 on behalf of the Ukrainian government to broker a meeting between Trump and the Ukrainian prime minister, Petro Poroshenko. Cohen, whose New York office and hotel room was raided by the FBI in April, served as Trump’s intermediary when he paid off the porn star Stormy Daniels on the eve of the 2016 election. The payment of $130,000 was to keep Daniels from discussing an alleged sexual encounter with Trump in 2006.

Cohen is being investigated for his work for a number of companies after Trump was elected, including AT&T, the Swiss pharmaceuticals firm Novartis and Columbus Nova, an investment firm with ties to a Russian oligarch.