House Speaker Nancy Pelosi on Sunday assailed the Trump administration's efforts to block a whistleblower complaint involving President Donald Trump's apparent effort to have Ukraine investigate former Vice President Joe Biden and his family.

In a letter to colleagues, Pelosi said the administration "will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation" if acting Director of National Intelligence Joseph Maguire fails to provide the complaint when he testifies in front of the House Intelligence Committee on Thursday. The complaint reportedly centers around Trump's July conversation with Ukrainian President Volodymyr Zelensky during which Trump is accused of pressuring Zelensky to investigate Biden's son Hunter's role at a Ukrainian energy company.

Pelosi's letter comes as frustration is increasing among lawmakers. They are frustrated not only with what they see as the president's obstruction of the congressional investigation but also with the slow-moving probe into him, lawmakers and aides say.

Frustration escalated after former Trump aide Corey Lewandowski's day of mostly stonewalling. On Friday, during a meeting of Judiciary Committee Democrats, two Democratic aides said committee leadership tried to calm members who were angry that the panel didn't hold him in contempt on the spot during the hearing.

As the New York Times and others reported, Trump's phone call led to a whistleblower complaint which is now at the center of a standoff between Congress and the administration. The administration has refused to turn over to Congress details of the complaint, though The Washington Post has reported that the allegation centers on a "promise" Trump made.

House Intelligence Committee Chairman Adam Schiff, D-Calif., questioned Sunday that, if Trump's conversation involved no wrongdoing, "why doesn't the president simply release the transcript of that call?"

"And I don't know whether the whistleblower complaint is on this allegation but if it is and even if it isn't, why doesn't the president just say 'release the whistleblower complaint?'" Schiff told CNN's "State of the Union." "Clearly, he's afraid for the public to see either one of those things and we're determined to make sure the public does, the nation is protected."

Administration officials said Sunday that such transcripts are not released so that foreign leaders and the president can discuss matters candidly, but Schiff said there should be no privilege afforded to discussions that "involve potential corruption or criminality."

"This would be the most profound violation of the presidential oath of office certainly during this presidency, which says a lot, but perhaps just about during any presidency," Schiff said. "There is no privilege that covers corruption. No privilege to engage in underhanded discussions."

Schiff added that the "only remedy" to such behavior is impeachment.

Former Secretary of State John Kerry, speaking with CBS's "Face the Nation," called the push for Ukraine to probe Biden a "fundamental, profound and deeply concerning abuse of power," adding it was "unprecedented" and that the transcript should be released to show whether Trump "colluded" with a foreign government to influence the upcoming presidential election.

On NBC's "Meet the Press," Sen. Chris Murphy, D-Conn., said that if the president "is asking another foreign leader to interfere in an American election," then "there has to be consequences."

Murphy, a member of the Senate Foreign Relations Committee, recently met with Zelensky in Kiev. He said the Ukrainian president "didn’t understand whether this was an official government position, these requests to investigate the former vice president."

Some Republicans also expressed concern about the phone call. Sen. Pat Toomey, R-Pa., told "Meet The Press" that while he didn't "know the context" of the phone call between Trump and Zelensky or "what was said," "it is not appropriate for any candidate" to "ask for assistance from a foreign government."

Sen. Mitt Romney, R-Utah, said in a Sunday tweet that if Trump "asked or pressured Ukraine’s president to investigate his political rival, either directly or through his personal attorney, it would be troubling in the extreme."

"Critical for the facts to come out," he said.

Meanwhile, Trump and his allies sought to deny any wrongdoing by the president. Responding to Romney in a text message to NBC News, the president's personal attorney, Rudy Giuliani, said Romney is "still trying to figure out what happened to him when that Candy lady whacked him around in the debate with" then-President Barack Obama, referencing a moment from a 2012 presidential debate moderated by former CNN anchor Candy Crowley.

Giuliani said the Ukrainian efforts were simply "what presidents do who care about the selling of public office."

And on the Sunday show circuit, Treasury Secretary Steven Mnuchin told "State of the Union" that he would not "speculate" on what Trump discussed with Zelensky, though he said he didn't "expect there were any issues whatsoever."

Mnuchin told host Jake Tapper: "I don't have any reason to believe that the president pressured," adding later "there is really no issue here." Releasing the transcript of that phone call "would be a terrible precedent," he said.

Secretary of State Mike Pompeo said on ABC's "This Week" that if Biden "behaved inappropriately, if he was protecting his son and intervened with the Ukrainian leadership in a way that was corrupt, I do think we need to get to the bottom of that." He said he'd allow the White House to explain why they have not released the transcript of the phone call, calling it "rare" that such a transcript would be released.

Trump told reporters Sunday that he did discuss Biden with Zelensky but said there was "absolutely nothing wrong" with the call. He told reporters later Sunday that he might provide a transcript of the conversation to a "respected source."

"The conversation I had was largely congratulatory, with largely corruption, all of the corruption taking place and largely the fact that we don't want our people like Vice President Biden and his son creating the corruption already in the Ukraine and Ukraine has got a lot of problems," Trump said. "The new president is saying that he's going to be able to rid the country of corruption, and I said that would be a great thing, we had a great conversation."

The president also said there was "no quid pro quo, there was nothing," and, "It was a perfect conversation."

In a campaign memo sent Sunday afternoon, Biden's deputy campaign manager Kate Bedingfield wrote that the Ukraine episode showed "Trump and his campaign are terrified of facing Vice President Biden in a general election."

"The president and his lackeys are so terrified of that prospect — and have abandoned any pretense of having an iota of truth on their side as they flail and try to distract from what may be the greatest presidential malfeasance since Watergate — that they fell all over themselves lying on the Sunday shows today," she added.

The president's focus on Biden and Ukraine comes amid Giuliani's monthslong effort to get Ukraine to further investigate the former vice president, an effort which was aided by the State Department, as NBC News reported last month. The push centers on Biden's 2016 call — widely backed by the international community — for Ukraine to crack down on corruption, including removing a Ukrainian prosecutor, Viktor Shokin, who was seen as ineffective and was later removed by the country's parliament. One of the cases that Shokin was investigating involved Burisma Holdings, a natural gas company whose board at the time included Biden's son.

Earlier this year, Bloomberg News, citing documents and an interview with a former Ukrainian official, reported the Burisma investigation had been dormant for more than a year by the time Biden called for the crackdown on corruption. The then-Ukrainian prosecutor general told the news agency he found no evidence of wrongdoing by Biden and his son. PolitiFact, meanwhile, reported that it found no evidence to "support the idea that Joe Biden advocated with his son's interests in mind."

House Democrats have already announced an investigation into Giuliani's Ukrainian efforts. On Wednesday, Trump is set to meet face to face with Zelensky on the sidelines of the United Nations General Assembly in New York. In the weeks before the whistleblower complaint became public, the Trump administration froze $250 million in military aid to Ukraine for unclear reasons. Then, just before Democrats revealed the existence of the whistleblower complaint, the administration released the hold on the Ukrainian military aid.

On Saturday, Ukrainian Foreign Minister Vadym Prystaiko said he didn't think Trump pressured Zelensky, a former comedian, during their July phone call.

"I know what the conversation was about, and I think there was no pressure," he said. "There was talk, conversations are different, leaders have the right to discuss any problems that exist. This conversation was long, friendly, and it touched on a lot of questions, including those requiring serious answers."

Speaking to reporters in Iowa on Saturday, Biden said "Trump's doing this because he knows I'll beat him like a drum and he's using the abuse of power and every element of the presidency to try to do something to smear me."