Washington (CNN) Lawyers for a whistleblower who accused Donald Trump of pressuring Ukraine to intervene in the 2020 election warn that the President's threats pose a grave risk to their client's safety.

Trump meanwhile escalated his attacks on the whistleblower -- demanding to meet his "accuser" face to face during a day of rage-filled tweets about the Democratic attempt to impeach him.The President's anger spilled over during a day of rage-filled tweets Sunday in which he selectively quoted a supporter who said he was afraid of a "Civil War-like fracture" in the country if Trump is forced from office.

The extraordinary spectacle of the President -- the titular head of the US legal system -- threatening a potential witness in a case against him risks being seen as an attempt to obstruct the investigation. It also cuts against the principle that whistleblowers deserve anonymity and protection, representing another dark twist in an administration that has constantly tested the boundaries of political propriety.

On another frenetic day of political exchanges, Democrats sought to engineer a fast start to their impeachment efforts as their chances of political success hinge on early momentum to keep the White House off balance.

Trump however led a ferocious fight back over the weekend, lashing out at Democrats, the media and the whistleblower as some of his top allies battled through a series of contentious appearances on Sunday talk shows.

On Sunday night, it emerged that lawyers for the whistleblower wrote to the chairmen and ranking members of the House and Senate Intelligence Committees and Director of National Intelligence Joseph Maguire to express "serious concerns" for their client.

"The events of the past week have heightened our concerns that our client's identity will be disclosed publicly and that, as a result, our client will be put in harm's way," the lawyers wrote in a letter date Saturday, September 28, before directly citing a comment by Trump last week.

On that occasion, the President said the person that gave the whistleblower the information was "close to a spy" and hinted at the possibility of execution for such behavior.

The lawyers noted that Trump was not referring directly to the whistleblower, but said that fact did not assuage their concerns and alleged that several unnamed parties had offered a $50,000 bounty for information on their client's identity.

"Unfortunately, we expect this situation to worsen, and to become even more dangerous for our client and any other whistleblowers, as Congress seeks to investigate this matter," the lawyers wrote in a letter obtained by CNN's Pamela Brown.

The letter from the lawyers emerged after House Intelligence Committee chairman Adam Schiff Sunday said he had reached a deal to secure testimony from the whistleblower. An attorney for the whistleblower said discussions were continuing.

Trump, meanwhile, kept up his bid to discredit the whistleblower.

"I want to meet not only my accuser, who presented SECOND & THIRD HAND INFORMATION, but also the person who illegally gave this information, which was largely incorrect, to the "Whistleblower." Was this person SPYING on the U.S. President? Big Consequences!" Trump tweeted Sunday.

The President also called for Schiff to be questioned for fraud and treason.

Trump also tweeted about the "Civil War-like fracture" comment by Robert Jeffress, a supporter on Fox News, prompting Republican Rep. Adam Kinzinger to offer a rebuke.

"I have visited nations ravaged by civil war. @realDonaldTrump I have never imagined such a quote to be repeated by a President. This is beyond repugnant," Kinzinger tweeted.

'Little Nancy Drew novel'

The White House, which seemed caught off guard in a wild week that saw House Speaker Nancy Pelosi open the impeachment inquiry , hit back hard in a bid to discredit allegations that the President abused his power by seeking dirt from Ukraine on a potential 2020 opponent, Joe Biden.

"The President of the United States is the whistleblower," senior Trump adviser Stephen Miller said on "Fox News Sunday."

"And this individual is a saboteur trying to undermine a democratically elected government," he said, blasting the complaint as a "little Nancy Drew novel."

But Pelosi told her caucus on a conference call Sunday that they should try to be non-partisan about the impeachment process.

"This isn't about politics. It's not about partisanship. It's about patriotism," she said.

"The idea that this has anything to do with whether you like (Trump) or not -- forget that. That's about the election. This is about the Constitution."

The speedy escalation of the political war was remarkable, given that it is only a week since Schiff declared Trump had "crossed the Rubicon" in his dealings with Ukraine President Volodymyr Zelensky.

Democrats and the White House are now locked in a historic confrontation that will test the US political system to its limits and will shape the destiny of the 2020 election.

Democrats train narrow focus on Ukraine

Democrats are under pressure to produce a concise impeachment probe that keeps a tight focus on Trump's alleged wrongdoing and makes a clear case to Americans.

To that end, Pelosi has made Schiff the primary face of the investigation after a Judiciary Committee hearing with Trump's ex-campaign manager Corey Lewandowski this month turned into a farce.

Speed is important because a failure to decide whether the full House will vote on Articles of Impeachment within months could overshadow Democratic primaries when the party's candidates hope to be talking about health care and economic inequality -- issues that preoccupy voters -- rather than Trump.

Still, any sign that Democrats are rushing could play into GOP claims that they have decided to impeach the President wherever the evidence leads.

While Republicans are unlikely to vote to convict Trump in a Senate trial, Democrats hope to persuade general election voters he's unfit for office and to heap pressure on GOP Senate candidates in swing states who may not relish a vote to acquit Trump.

But the longer Trump can drag the impeachment intrigue out, the better it may be for his political fortunes.

With time, he can crank up efforts to discredit the impeachment probe with the help of the conservative media machine - a tactic that worked well in shaping perceptions of the Mueller probe. The President will also seek to change the subject -- perhaps with big ticket foreign policy goals that could bolster his argument that he's doing the people's business and should not be impeached.

The White House will also likely make expansive executive privilege claims and fight in court to frustrate Democrats and build public frustration with the impeachment saga. And the President can put claims that Democrats are trying to subvert the result of the 2016 election at the center of his rallies to fire up his loyal supporters as 2020 approaches.

Action looming on Capitol Hill

As the pace heats up in Washington, former US special envoy to Ukraine Kurt Volker — who resigned last week — plans to appear for a deposition before three House committees on Thursday.

Volker is a long-time Republican foreign policy hand who was close to late Sen. John McCain — the most prominent GOP critic of the President. This could be a wild card moment for the White House, which may seek to exert privilege over the ambassador's dealings with Ukraine on behalf of Trump.

Schiff said Sunday that he also expects the whistleblower to testify "very soon" following negotiations focusing partly on how to preserve his or her anonymity and security.

"We will get the unfiltered testimony of that whistleblower," he said on ABC News "This Week."

CNN reported on Wednesday the potential testimony is dependent on the whistleblower's attorneys getting security clearances.

In the complaint, judged credible by a Trump-appointed intelligence community inspector general, the whistleblower alleged the President tried to get Ukraine to interfere in the 2020 election and the White House tried to cover it up.

Trump has repeatedly denied that he did anything wrong, saying his call with Zelensky was "perfect." He has also dismissed allegations he threatened to withhold US military aid to Ukraine if Kiev refused to investigate Biden.

Trump supporters were out in force on the Sunday talk shows.

Ohio Rep. Jim Jordan, in a contentious exchange with Jake Tapper on CNN's "State of the Union" tried to make the issue about Biden and his son Hunter who was on the board of a Ukrainian energy firm when his father was vice president.

"When the company that's paying that money is under investigation, guess what? Daddy comes running to the rescue. The vice president of the United States comes running in and says, 'Fire that prospector,'" Jordan said.

JUST WATCHED Tapper to Jordan: 'It's not gymnastics. It's the facts.' Replay More Videos ... MUST WATCH Tapper to Jordan: 'It's not gymnastics. It's the facts.' 03:29

Tapper replied: "That's not what happened. Sir, sir, that's not what happened," adding that the European Union, the International Monetary Fund and activists in Ukraine all wanted the prosecutor dismissed because he was not pursuing corruption.

There is no evidence that Biden or his son did anything wrong.

There was one note of criticism for Trump from an unexpected quarter on Sunday. Tom Bossert, who served as White House homeland security chief, faulted the President's handling of Ukraine.

"I'm deeply disturbed by it as well and this entire mess has me frustrated," Bossert, now an ABC contributor, told "This Week."

"I hope that everyone can sift through the evidence and be very careful, as I've seen a lot of rush to judgment this week. That said, it is a bad day and a bad week for this president and for this country if he is asking for political dirt on an opponent."

The frenzied nature of Sunday's exchanges reflected the fact that no one in Washington can really predict the political impact of the impeachment drama to come.

After all, conventional wisdom on the likelihood of such a process and the wisdom of political maneuvers by Trump and Pelosi shifted several times last week. The only certainty is that Washington is entering a fraught period that could make the vitriol of the last two-and-a-half years sound like a warmup.