WASHINGTON -- House and Senate lawmakers are set to meet with top intelligence officials as President Donald Trump raises new suspicions about the federal investigation into his 2016 campaign. Mr. Trump has dubbed his latest attempt to discredit special counsel Robert Mueller's Russia investigation "spygate."

In recent days, he has been zeroing in on -- and at times embellishing -- reports that a longtime U.S. 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.

Mr. Trump tweeted Wednesday that the FBI had been caught in a "major SPY scandal."

SPYGATE could be one of the biggest political scandals in history! — Donald J. Trump (@realDonaldTrump) May 23, 2018

Mr. Trump's latest broadsides set the stage for the unusual decision by the White House to arrange a briefing Thursday about classified documents for just two Republican House members, both Trump allies, as Mr. Trump and his supporters in Congress pressed for information on the outside informant.

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After Democratic complaints and negotiations that went into the late evening Wednesday, the Justice Department said it would host a second classified briefing the same day 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 some late additions to the list -- White House chief of staff John Kelly and Deputy Attorney General Rod Rosenstein. White House press secretary Sarah Huckabee Sanders had originally said that no one from the White House would attend the briefing, at which the investigation into Mr. Trump's campaign will be discussed.

Rosenstein was left off the list as Mr. Trump on Tuesday declined to say whether he had confidence in him. Rosenstein appointed special counsel Robert Mueller, who is leading the Russia investigation and is frequently criticized by Mr. Trump.

The two House lawmakers -- Intelligence Committee Chairman Devin Nunes and Oversight and Government Reform Chairman Trey Gowdy -- were invited to attend both briefings, as were Kelly, Rosenstein, FBI Director Christopher Wray and National Intelligence Director Dan Coats.

All were invited to the second briefing, as well, plus Senate Majority Leader Mitch McConnell, Senate Democratic leader Chuck Schumer, House Speaker Paul Ryan and House Democratic leader Nancy Pelosi. Senate Intelligence Committee Chairman Richard Burr was also invited, along with the top Democrat on the Senate intelligence panel, Sen. Mark Warner, and the top Democrat on the House intelligence panel, Rep. Adam Schiff.

On Thursday, House Democratic leader Nancy Pelosi said that the top Democrat on the House Intel Committee, Rep. Adam Schiff, D-Calif., would be attending both briefings to ensure that the same information is shared with both groups. Pelosi, too, was invited to the first meeting after she protested but decided not to attend because she thinks the meeting is inappropriate. Schiff is going in her stead.

A spokeswoman for Ryan told CBS News he would not be able to attend due to previous commitment, and that, "Chairmen Gowdy and Nunes will continue to lead in this space for House Republicans."

Nunes, an ardent supporter of President Trump, had originally requested the information on an FBI source in the Russia investigation. And Mr. Trump took up the cause as the White House tried to combat the threat posed by Mueller's investigation into Russian interference and possible obstruction of justice.

Look how things have turned around on the Criminal Deep State. They go after Phony Collusion with Russia, a made up Scam, and end up getting caught in a major SPY scandal the likes of which this country may never have seen before! What goes around, comes around! — Donald J. Trump (@realDonaldTrump) May 23, 2018

Mr. Trump escalated his efforts to discredit the investigation Wednesday, tweeting: "Look how things have turned around on the Criminal Deep State. They go after Phony Collusion with Russia, a made up Scam, and end up getting caught in a major SPY scandal the likes of which this country may never have seen before! What goes around, comes around!"

It remained unclear what, if any, spying was done. The White House gave no evidence to support Trump's claim that the Obama administration was trying to spy on his 2016 campaign for political reasons. It's long been known that the FBI was looking into Russian meddling during the campaign and that part of that inquiry touched on the Trump campaign's contacts with Russian figures. Mueller later took over the investigation when he was appointed in May 2017.

Trump has told confidants in recent days that the revelation of an informant was potential evidence that the upper echelon of federal law enforcement had conspired against him, according to three people familiar with his recent conversations but not authorized to discuss them publicly. Trump told one ally this week that he wanted "to brand" the informant a "spy," believing the more nefarious term would resonate more in the media and with the public.

As Republicans worked to show a Justice Department conspiracy against Trump, Democrats and former law enforcement officials defended the agency. Former FBI Director James Comey, who was fired by Trump last year, tweeted Wednesday that the agency's use of secret informants was "tightly regulated and essential to protecting the country."

"Attacks on the FBI and lying about its work will do lasting damage to our country," Comey tweeted. "How will Republicans explain this to their grandchildren?"

Trump shot back during a brief news conference with reporters: "What I'm doing is a service to this country and I did a great service to this country by firing James Comey."

The back and forth between Congress and the Justice Department over the Nunes request - one of many over the course of the Russia investigation - has simmered for weeks.

The department originally rejected Nunes' appeal, writing in a letter in late April that his request for information "regarding a specific individual" could have severe consequences, including potential loss of human life. Negotiations over the information stalled, but restarted when Trump demanded in a tweet Sunday that the Justice Department investigate "whether or not the FBI/DOJ infiltrated or surveilled the Trump Campaign for Political Purposes."

The Justice Department agreed by expanding an open, internal investigation to determine whether there was any politically motivated surveillance. And the White House said Kelly would organize the meeting with House lawmakers to discuss the documents.

The New York Times was the first to report that the FBI had an informant who met several times with Trump campaign officials who had suspicious contacts linked to Russia. No evidence has emerged to show that Obama-era authorities placed an informant inside the Trump campaign.