(CNN) A smoldering Washington intelligence mystery exploded late Wednesday into a full-on confrontation between House Democrats and the White House over communications between President Donald Trump and an unidentified foreign leader.

A showdown over a whistleblower's "credible and urgent" complaint was revealed -- first by The Washington Post and later confirmed by CNN -- to concern the President himself, raising new questions about the motivation of the acting director of national intelligence's refusal to reveal details of the case to Congress.

The revelations have unleashed a new firestorm in the nation's capital over Trump's perplexing foreign policy dealings and the administration's across-the-board efforts to frustrate Congress' constitutionally authorized role of oversight of the executive branch.

But they will also open debate about the extent of the President's powers, which give him expansive latitude in national security.

The whistleblower's claims are also certain to bolster the belief among Trump and his supporters that nefarious forces are operating within America's intelligence establishment to undermine him -- a refrain conservatives have used in the wake of the Russian election interference operation. And they will likely further damage the President's trust in America's spy agencies.

The Post reported that an official in the American intelligence community was so bothered by a "promise" Trump had made while communicating with a foreign leader that the official filed a formal whistleblower complaint with the inspector general of the intelligence community, according to two former US officials familiar with the matter. CNN has not confirmed the detail about the "promise."

It is unclear to whom Trump was speaking at the time.

The President dismissed the reporting on Thursday morning, rhetorically asking if there is "anybody dumb enough to believe" it.

"Virtually anytime I speak on the phone to a foreign leader, I understand that there may be many people listening from various U.S. agencies, not to mention those from the other country itself. No problem!," he tweeted.

"Knowing all of this, is anybody dumb enough to believe that I would say something inappropriate with a foreign leader while on such a potentially 'heavily populated' call. I would only do what is right anyway, and only do good for the USA!" Trump said.

The complaint was filed on August 12. White House records show Trump had spoken to or interacted with five foreign leaders in the previous five weeks, the Post reported: Russian President Vladimir Putin, North Korean dictator Kim Jong Un, Pakistani Prime Minister Imran Khan, Dutch Prime Minister Mark Rutte and the Emir of Qatar.

Trump's private, in-person meetings with Putin and Kim especially have intrigued observers.

The Post's report, and CNN's subsequent confirmation, shed light on an intrigue that puzzled Washington for the last few days.

The drama centers on the refusal of acting Director of National Intelligence Joseph Maguire to hand over the whistleblower's complaint to Congress. The stalling had already prompted speculation that Trump or senior aides could be involved.

The inspector general, Michael Atkinson, told the House Intelligence Committee on Thursday in a closed hearing that he was not allowed to provide details of the complaint, sources told CNN's Manu Raju.

Under the law, the substance of a whistleblowers' claims cannot be provided until the report is sent to Congress. Democrats argue that the DNI is breaking the law by refusing to pass on the complaint, an accusation Maguire's office denies.

After a standoff with House Intelligence Chairman Adam Schiff, Maguire agreed Wednesday to testify next week, in a public hearing about the affair. There is no indication that he will hand over the report -- especially in an open hearing.

Schiff had sent rumors into overdrive on Sunday when he said Maguire had cited a "higher authority" in refusing to hand over the complaint and referred to "privileged communications."

"I think it's fair to assume this involves either the President or people around him or both," the California Democrat said on CBS' "Face the Nation."

Thwarting oversight

The reports will trigger a controversy about Trump's handling of intelligence and his opaque conversations with foreign leaders.

A political duel will also rage about the whistleblower's decision to make the complaint. On one hand, the President's sweeping powers in foreign policy conferred by the Constitution and a democratic mandate give him wide latitude to say whatever he likes to a foreign leader. American presidents have been making promises to their counterparts on the world stage for generations. It's a practice that might be said to be part of their job.

Yet the exception would be if a president were engaging in contact antithetical to the interests of United States or carrying out illegal or unconstitutional acts that contravened his oath of office. There is no indication that is the case in the current controversy.

Former FBI and CIA official Phil Mudd, who is now a CNN analyst, on Wednesday argued that it was unfair to expect a senior intelligence official to reveal sensitive information about a president's conversations to Congress and it was not the job of America's spies to monitor him.

"Can you explain to me a) why it is the US intelligence community's responsibility to listen to the President of the United States speaking to a foreign leader and b) why it is the US intelligence community, under the rules provided by the Democrats in Congress, (who) are responsible to report to the Congress what the President of the United States says?" Mudd asked.

"When I served, we were responsible for chasing the Russians, the Chinese, the Iranians and terrorists," Mudd said on "Cuomo Prime Time."

"We are not responsible for reporting to the Congress what the President says. He can say what he wants."

Blocking Congress

Viewed another way, Maguire's conduct appears to be the latest manifestation of the administration's multi-front strategy to thwart Congress' oversight efforts.

If an inspector general in a government department finds misconduct or uncovers information that rises to a high level of wrongdoing defined as "urgent and credible" under whistleblowing legislation, the legal recourse is to involve Congress.

The controversy came to a head only a day after Trump's former campaign manager Corey Lewandowski put on an aggressive display in a House Judiciary Committee hearing that prompted the White House to preview a sweeping executive privilege claim even though he never worked for the federal government.

The White House has sought to prevent testimony from many other current and former executive branch officials, leading to a string of looming court clashes that in themselves fulfill Trump's goal of slowing Democratic investigations.

Maguire is a retired Navy admiral who formerly led the National Counterterrorism Center. He was chosen by Trump following a succession drama after the permanent director -- former Sen. Dan Coats, who was never seen as close to the President -- announced he would step down in July. Trump passed over the office's No. 2, Sue Gordon . Sources told CNN at the time she was not seen as the kind of loyalist Trump wanted in the role.

At the time, Maguire was seen as a force for stability. Trump's first pick to replace Coats, Rep. John Ratcliffe, stepped down after controversy over the Texas Republican's qualifications and amid concern he would be a political appointee who would first be loyal to the President, not his duty as director of national intelligence.

Witnesses armed with letters

Schiff announced Wednesday that Maguire will appear before the House Intelligence Committee at 9 a.m. on September 26.

His comment over the weekend that Maguire was citing privilege considerations raises the prospect that the committee could see a replay of a now-familiar scenario.

Witnesses are increasingly showing up at hearings -- like Lewandowski -- armed with letters from the White House lawyers advising them to limit their testimony to issues not likely to be involved in expansive future executive privilege claims.

The California Democrat also doesn't know the identity of the whistleblower.

The acting director's letter Tuesday to Schiff, obtained by CNN, states that the complaint does not involve anyone in the intelligence community but rather "stakeholders within the Executive Branch." As a result, its lawyer argues, the complaint is not of "urgent concern" to the office.

The Senate Intelligence Committee is also getting involved in the case.

The panel's top Democrat, Sen. Mark Warner of Virginia, told reporters that he and his Republican counterpart "want to get answers. We need to protect whistleblowers and we hope to get some resolution next week."

Its chairman, Sen. Richard Burr, a North Carolina Republican, earlier declined to comment, but, while saying he would follow the whistleblower's progress, added, "That's not typically something that Congress gets involved in."