WASHINGTON (REUTERS) - For nearly three years, United States President Donald Trump has dismissed the investigations that have shadowed almost every day of his presidency as hoaxes and witch-hunts, and the people leading them as crooks and liars.

But, so far, none of his targets has produced a public display of wrath to equal what the President directed at his latest antagonist, US Representative Adam Schiff.

The Democratic lawmaker, chairman of the House Intelligence Committee, has become the public face of a rapidly escalating impeachment inquiry that is the latest and perhaps most serious threat to Mr Trump's presidency.

Mr Trump, who has branded the impeachment probe a "hoax", has attacked Mr Schiff as a "lowlife" and a liar, thundered at him during a White House news conference and suggested that he be questioned or arrested for treason.

"Schiff is a lowlife who should resign (at least!)," Mr Trump wrote on Twitter on Thursday (Oct 3). Hours later, on the White House lawn, he described Mr Schiff as a "stone-cold liar".

Mr Schiff said in a statement that Mr Trump "would much rather attack others than answer for his own conduct".

Some of the President's allies have echoed his attacks on Mr Schiff's credibility. Republican strategists have also been distributing talking points targeting him, a person familiar with the party's election campaign told Reuters.

Mr Trump sees Mr Schiff's involvement as an avenue for attacking the impeachment probe because he had already been an outspoken critic of the President, suggesting the public anger and attacks are at least partly strategic.

"Trump's job is to break up the narrative and create doubt," the person said.

House Democrats launched an impeachment inquiry in September after an unidentified whistle-blower alleged Mr Trump had sought to pressure his Ukrainian counterpart to investigate former US vice-president Joe Biden, a leading contender for the 2020 Democratic presidential nomination.

During a July 25 call, Mr Trump asked Ukrainian President Volodymyr Zelensky to speak with the Republican president's personal lawyer Rudy Giuliani. The former New York mayor has been pursuing a globe-trotting effort to find out whether Ukrainian officials improperly dropped an investigation of a company that hired Mr Biden's son, Hunter Biden.

The inquiry has produced an intense partisan fight, and a flood of political advertising to match. But little of that has been directed at Mr Schiff.

The largest super PAC (political action committee) backing Mr Trump, America First Action SuperPAC, is not planning to target any of its advertising campaign at Mr Schiff, the person familiar with Republicans' strategy said.

Instead, the super PAC, an outside group that can spend unlimited amounts of money to support candidates, plans to focus on Democrats in House districts that Mr Trump won in 2016.

The group does not rule out targeting Mr Schiff later, but is unlikely to do so unless the US Congress moves closer to an impeachment vote, the person said.

ATTACK ADS

On Facebook, only a handful of ads have targeted Mr Schiff.

House Minority Leader Kevin McCarthy, a Republican from California, spent about US$100 (S$138) for an ad claiming Mr Schiff had put the country "through a nightmare". It reached fewer than 1,000 people, according to data Facebook published about the ad.

Mr Schiff, who represents a Los Angeles district, is a former federal prosecutor and a political ally of House Speaker Nancy Pelosi. His public profile grew over the past two years as a sharp-spoken but seldom-ruffled defender of the investigation of Russian meddling in the 2016 US election.

Representative Gerry Connolly, a Democrat from Virginia, said Mr Schiff was a good choice to lead the impeachment probe because he could withstand the President's attacks and is "not going to be deterred from doing his job".

"We're not fooling around here," Mr Schiff said at a news conference with Ms Pelosi discussing the impeachment probe. "We don't want this to drag on for months and months and months, which appears to be the administration's strategy."

His approach has drawn harsh critiques from Republican lawmakers. In March, two months after Mr Schiff took over as chairman of the House committee, all nine of the panel's Republican members demanded he resign over his allegations that Mr Trump's campaign had colluded with Russia in the 2016 campaign.

An investigation by US Special Counsel Robert Mueller concluded that the Russian government sought to sway the election in Mr Trump's favour and that his campaign worked to benefit from those efforts, but did not find evidence to establish a conspiracy.

Some Republican lawmakers repeated those calls this week.

Representative Elise Stefanik, a Republican from New York, said in a statement that Mr Schiff had "made a mockery of our Committee and should step down as Chairman".

Ms Stefanik, other Republican lawmakers and Mr Trump focused their latest attacks on Mr Schiff's opening statement during a committee hearing last week, when he delivered a dramatised interpretation of Mr Trump's call with Mr Zelensky that he said later in the hearing was "meant to be at least part in parody".

INTENSIFYING BATTLE

Mr Schiff's remarks produced escalating public battles with the President.

A week ago, Mr Trump dismissed Mr Schiff as "Liddle' Adam Schiff". Mr Trump accused him on Sunday of lying to Congress and tweeted that he should be "questioned at the highest level for Fraud & Treason". The next day, Mr Trump added on Twitter: "Arrest for Treason?"

Mr Schiff emerged as the face of the Democrats' impeachment inquiry in part because the President's call with Ukraine would have come to the House Intelligence Committee, and in part because Democratic lawmakers have been frustrated with how a half-dozen other committees have carried out their own investigations of the President.

The White House has largely stonewalled investigations by Democratic-led committees, a slow-motion clash that seemed to reach its apex when Mr Corey Lewandowski, Mr Trump's former campaign manager and a close confidant, spent five hours of public testimony trading insults and accusations with lawmakers on the House Judiciary Committee and answering few of their questions.

Democrats have been insistent on not repeating that performance.

A senior House Democratic aide said freshmen House Democrats from competitive districts, whose opinions have proven pivotal to the impeachment process, gave Mr Schiff good reviews for his handling of the probe during a closed-door meeting on the impeachment process with Ms Pelosi and other Democratic leaders last week.

"They didn't want another Lewandowski hearing, that was a recurrent theme," the aide said.