A top House Republican said Sunday that he will demand testimony from House intelligence committee Chairman Adam B. Schiff as the impeachment inquiry heads toward its next phase, keeping Republicans’ focus squarely on a process they dismiss as a partisan attack on President Trump.

Rep. Doug Collins of Georgia, the top Republican on the House Judiciary Committee, said he wants to call the high-ranking California Democrat because his side has no guidance ahead of a Wednesday hearing on possible articles of impeachment.

Mr. Schiff will release a report Monday on what the House Permanent Select Committee on Intelligence learned from fact witnesses and vote on it Tuesday, giving Judiciary Committee Republicans little time to digest his interpretation and form their witness list, Mr. Collins said.

“The first person that needs to testify is Adam Schiff. Adam Schiff is the author of this report,” he told “Fox News Sunday.”

Republicans have singled out Mr. Schiff as the architect of a “sham” process that began with an anonymous whistleblower complaint in September, though Democrats say Mr. Trump needs to be more forthcoming about behavior they insist is more damaging and unethical than the Nixon administration’s actions during the Watergate scandal.

House Judiciary Committee Chairman Jerrold Nadler, the New York Democrat who will draft articles of impeachment, has given Mr. Trump until Friday to decide whether he wants to participate in proceedings. The White House prevented key witnesses from appearing before Mr. Schiff.

Judiciary Committee Democrats on Sunday said they haven’t prejudged Mr. Trump’s conduct and would like to see evidence that clears him of wrongdoing if it exists, though they remain doubtful.

“It would be wonderful if there were some benign explanation. I’m struggling to think what it would be at this point,” Rep. Zoe Lofgren, California Democrat, told CNN’s “State of the Union.”

Sen. Amy Klobuchar of Minnesota said it’s unlikely she will vote to acquit the president if and when she leaves the campaign trail to serve as a juror in a Senate trial.

“I can’t see that happening right now, but I am someone that looks at each count and makes a decision,” the Democratic presidential candidate told NBC’s “Meet the Press.” “I’ve said from the beginning I see this as impeachable conduct.”

She said Mr. Trump’s actions amount to a “global Watergate,” only the White House tapped into Ukraine instead of a file cabinet in Washington.

The Capitol Hill fight will unfold as Mr. Trump jets to London this week for meetings with NATO leaders.

Mr. Trump, who spent the holiday weekend in Florida, castigated Democrats on Twitter for focusing on impeachment while he pushes a North American trade deal and showcases his day-to-day work as president, including a surprise visit to Afghanistan to spend Thanksgiving with U.S. troops.

“I will be representing our Country in London at NATO, while the Democrats are holding the most ridiculous Impeachment hearings in history,” he tweeted over the weekend.

The impeachment inquiry stems from a July 25 phone call in which Mr. Trump asked Ukrainian President Volodymyr Zelensky for assistance in investigating matters involving Democrats and former Vice President Joseph R. Biden, a potential 2020 challenger.

A whistleblower, who is believed to be a CIA official assigned to the White House, accused the president of abusing his power for personal gain on the call, including withholding nearly $400 million of U.S. military aid from Ukraine as leverage.

A rough transcript of the call the White House released in late September did not show the president offering a quid pro quo deal for the investigations, but Democrats have argued that the threat was understood and part of an ongoing pressure campaign of “shadow” foreign policy conducted by Mr. Trump’s private attorney, former New York Mayor Rudolph W. Giuliani.

The quid pro quo, a Latin term meaning a transaction of “this for that,” is the crux of the Democrats’ case that Mr. Trump engaged in a bribery or extortion scheme that warrants impeachment.

The most powerful testimony yet in the inquiry came from Gordon Sondland, the U.S. ambassador to the European Union, who said he offered a quid pro quo of a White House visit for the newly elected Mr. Zelensky in exchange for the investigations. But Mr. Sondland said he “presumed” that was what Mr. Trump wanted.

In his only conversation with the president about the matter, Mr. Trump told him that there was “no quid pro quo,” he testified.

So far, none of the witnesses in the impeachment inquiry has linked the holdup of military assistance to the investigations or provided a reason for the delay. The aid was on hold for about two months and started to flow to Ukraine on Sept. 11, two days after the inspector general of the intelligence community informed Congress of the whistleblower complaint.

Mr. Trump has acknowledged that he wanted an investigation into suspected corruption involving Mr. Biden and his son Hunter, who landed a high-paying job on the board of Ukrainian natural gas company Burisma Holdings in 2014. At the time, his father was the point man for the Obama White House on Ukraine, which is notorious for corruption.

The former vice president later boasted of forcing Ukrainian leaders to fire the country’s chief prosecutor in spring 2016 by threatening to block a $1 billion U.S. loan guarantee. The prosecutor was widely viewed as not doing enough to combat corruption, but the prosecutor, Viktor Shokin, had looked into corruption allegations against Burisma and Mykola Zlochevsky, the Ukrainian oligarch running the company.

Mr. Trump also wanted Ukraine to look into a missing Democratic National Committee server that was hacked by Russia during the 2016 presidential campaigns. An American cybersecurity company called CrowdStrike examined the server to investigate the hack.

Mr. Trump subscribes to an unsubstantiated theory that the server ended up in Ukraine.

Sen. John Kennedy, Louisiana Republican, recently had to walk back an assertion that Ukraine may have been responsible for hacking the DNC server.

On Sunday, Mr. Kennedy said he meant to say that Ukraine meddled more generally in the 2016 election.

Mr. Kennedy cited multiple outlets that reported on Ukrainian President Petro Poroshenko’s push to elevate Hillary Clinton over Mr. Trump, whom he saw an unfit for the office.

“The fact that Russia was so aggressive does not exclude the fact that President Poroshenko actively worked for Secretary Clinton,” he told NBC’s “Meet the Press.”

He said the impeachment inquiry into Mr. Trump is “as rigged as a carnival ring toss.”

Mr. Collins waded briefly into the substance of Mr. Trump’s actions, saying the president has long been wary of helping other nations without guarantees that the money will be well spent.

“The president told Sen. [Ron] Johnson[, Wisconsin Republican,] there was no precondition,” Mr. Collins said. “President Trump has always been concerned about foreign aid.”

But Democrats say Mr. Trump invited a foreign government to interfere in domestic politics, violating tenets that date to America’s founding.

“If you take a look at what the Founding Fathers were concerned about, it was the interference by foreign governments in our political system. That was one of their gravest concerns,” Ms. Lofgren said.

Although President Nixon was caught using the levers of government power for ill, she said, “at least it didn’t involve involving other foreign nations.”

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