WASHINGTON — House Intelligence Committee Chairman Adam Schiff said Sunday that he expects the whistle-blower at the heart of impeachment proceedings against President Donald Trump to testify “very soon.”

“All that needs to be done, at this point, is to make sure that the attorneys that represent the whistle-blower get the clearances that they need to be able to accompany the whistle-blower to testimony,” said Schiff, D-Calif., “and that we figure out the logistics to make sure that we protect the identity of the whistle-blower.”

As Democrats and the director of national intelligence worked out key arrangements, Trump’s allies erupted in a surge of second-guessing and conspiracy theorizing across the Sunday talk shows, suggesting the White House strategy is unclear against the stiffest challenge to his presidency. One former adviser urged Trump to confront the crisis at hand and get past his fury over the probe of Russian election interference.

“I honestly believe this president has not gotten his pound of flesh yet from past grievances on the 2016 investigation,” said Tom Bossert, Trump’s former Homeland Security adviser. “If he continues to focus on that white whale,” Bossert added, “it’s going to bring him down.”

The Ukraine investigation produced what the Russian probe did not: formal House impeachment proceedings based on the president’s own words and actions.

The White House last week released a rough transcript of Trump’s July 25 call with Ukranian President Volodymyr Zelenskiy, as well as the whistle blower’s complaint alleging the U.S. president pressured his counterpart to investigate the family of Joe Biden, the former vice president who is seeking the Democratic nomination to challenge Trump’s reelection next year.

In a series of tweets Sunday night, Trump said he deserved to meet “my accuser” as well as whoever provided the whistle-blower with what the president called “largely incorrect” information. He also accused Democrats of “doing great harm to our Country” in an effort to destabilize the nation and the 2020 election.

Trump has sought to implicate Biden and his son Hunter Biden in the kind of corruption that has long plagued Ukraine. Hunter Biden served on the board of a Ukrainian gas company at the same time his father was leading the Obama administration’s diplomatic dealings with Kyiv. There has been no evidence of wrongdoing by either of the Bidens.

The House forged ahead, with Schiff’s committee leading the investigation. Democrats are planning a rapid start to their push for impeachment, with hearings and depositions starting this week. Many Democrats are pushing for a vote on articles of impeachment before the end of the year, mindful of the looming 2020 elections.

Schiff has said the whistle-blower has agreed to testify, but the logistics involving security had yet to be set.

On a conference call later Sunday, Pelosi, who was traveling in Texas, urged Democrats to proceed “not with negative attitudes toward him, but a positive attitude toward our responsibility,” according to an aide on the call who requested anonymity to share the private conversation. She also urged the caucus to be “somber” and noted that polling on impeachment has changed “drastically.”

A one-day NPR/PBS NewsHour/Marist poll conducted last Wednesday found that about half of Americans — 49 percent — approve of the House formally starting an impeachment inquiry into Trump.

There remains a stark partisan divide on the issue, with 88 percent of Democrats approving and 93 percent of Republicans disapproving of the inquiry.

On the call, Democratic Caucus Chairman Hakeem Jeffries of New York urged the caucus to talk about impeachment by repeating the words “betrayal, abuse of power, national security.”

In contrast, Republicans offered a televised array of strategies to a president who spent the day at his golf club in Virginia and prefers to handle his own communications.

Stephen Miller, the president’s senior policy adviser, called the whole inquiry a “partisan hit job” orchestrated by “a deep state operative” who is also “a saboteur.”

“The president of the United States is the whistle-blower,” Miller said.

Rudy Giuliani, Trump’s personal lawyer who has been encouraging Ukraine to investigate both Biden and Hillary Clinton, promoted a debunked conspiracy theory, insisting that Ukraine had spread disinformation during the 2016 election.

Schiff said in one interview that his committee intends to subpoena Giuliani for documents and may eventually want to hear from Giuliani directly. In a separate TV appearance, Giuliani said he would not cooperate with Schiff, but then acknowledged he would do what Trump tells him.