Trump claims 'you'll see it' when asked for proof of alleged 'spying' scandal "Look how things have turned around on the Criminal Deep State," Trump tweeted.

As he left the White House Wednesday headed to New York, President Donald Trump kept pounding away at what he's now calling the "SPYGATE" scandal after a Twitter tear on the unsubstantiated allegations earlier in the morning.

Asked by reporters what proof he had that his campaign was spied on, the president replied: "All you have to do is look at the basics and you'll see it. Looks like a very serious event, but we'll find out."

Trump referenced classified documents the Justice Department is scheduled to show two GOP congressmen Thursday after Trump issued a "demand" Sunday that DOJ investigate what he then called possible efforts by the FBI to "surveil" and "infiltrate" his 2016 campaign.

Republicans lawmakers have been in a long-running battle with the Justice Department over the documents - claiming they could show possible DOJ misconduct.

"When they look at the documents, I think people are going to see a lot of bad things happen, " Trump said. "I hope it's not so, because if it is, there's never been anything like it in the history of our country... But I hope it's not true, but it looks like it is."

In his tweets earlier Wednesday, the president said, "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!"

His tweet appears to reference New York Times and Washington Post reports that the FBI used an informant to make contact with members of his campaign, only after the agency obtained information that members of the Trump team had suspicious contacts with Russians during the 2016 election.

In recent days the president, in tandem with his allies in conservative media, has launched a full frontal attack on special counsel Robert Mueller's probe into Russian election meddling and possible collusion with the Trump campaign. Earlier this week, he called for an investigation of Mueller's investigation and alleged the FBI spied on his campaign, a story he described as “bigger than Watergate.” But the president has provided no evidence to support his allegations.

Former FBI Director James Comey, whom Trump fired just over a year ago, tweeted a response Wednesday to the president's attacks on the FBI, calling them lies.

"Facts matter," Comey wrote. "The FBI’s use of Confidential Human Sources (the actual term) is tightly regulated and essential to protecting the country. Attacks on the FBI and lying about its work will do lasting damage to our country. How will Republicans explain this to their grandchildren?"

Comey was FBI director when the bureau's investigation into Russian election meddling began and would have had authority over reported contacts between FBI informants and members of the Trump campaign.

In a second tweet, Comey said it's a "dangerous time when our country is led by those who will lie about anything, backed by those who will believe anything, based on information from media sources that will say anything. Americans must break out of that bubble and seek truth.”

Trump, in his tweets, said, "SPYGATE could be one of the biggest political scandals in history!"

This latest unproven claim of a spy scandal now joins of list of other bold allegations by the president that so far have not proven true, including his claim that President Barack Obama had Trump Tower wiretapped, that millions of people voted illegally in the 2016 election, and that President Obama was not born in the United States.

Meanwhile, the Department of Justice announced earlier this week that the president's demand for an investigation into the FBI's investigation will be handled as part of an ongoing internal review by the department's inspector general. And officials from the Justice Department, Director of National Intelligence and the FBI are expected to comply Thursday with a request from a handful of House Republicans to brief them on highly classified documents related to the FBI probe.

These lawmakers, led by House Intelligence Chairman Devin Nunes, R-Calif., hope the documents will answer their questions about whether the FBI overstepped. But the top Democrat on the House Intelligence Committee, Rep. Adam Schiff, D-Calif., calls it an abuse of power.

"I don’t think any of us have any idea what the White House is doing, except that they want to use any mechanism they can to get their hands on materials they think will be useful for their legal defense team and they’re willing to break down the wall of independence between the White House and the Justice Department to do it," Schiff told ABC News. "Sadly, they have allies in Congress who are all too happy to help in destroying these institutions."

The Senate's top Democrat, New York’s Chuck Schumer, blasted the planned Thursday meeting between the DOJ, FBI, DNI and Republican congressional leaders, saying he and House Democratic Leaders Nancy Pelosi of California are sending a letter to Deputy Attorney General Rod Rosenstein and FBI Director Christopher Wray Wednesday to request it reconsider holding the meeting.

Schumer said in the letter that if the meeting is held at all, it should include the congressional "Gang of 8" to ensure it is bipartisan and inclusive. The "Gang of 8" includes the Republican and Democratic leaders from the House and Senate as well as the respective party leaders from the House and Senate intelligence committees.

"If the meeting goes forward as planned right now, only partisan, only the worst actors in the House side of the room, no one should trust anything coming out of that meeting,” Schumer said. “It will be a sham.”

Schumer implored Rosenstein and Wray to remember their responsibility to be impartial, saying he fears they're being pushed around by the White House and its allies, including chairman Nunes.

ABC News' Jordyn Phelps contributed to this report.