House Intelligence Committee Chairman Adam Schiff, D-California, with committee staffer Daniel Noble at left, makes impassioned remarks at the conclusion of a week of public impeachment hearings on President Donald Trump's efforts to tie U.S. aid for Ukraine to investigations of his political opponents on Capitol Hill in Washington on Thursday. Credit: J. Scott Applewhite | AP

House Intelligence Committee Chairman Adam Schiff, D-California, said Sunday that his panel will press ahead with its impeachment report even though key witnesses have not testified, in the latest signal that Democrats are moving swiftly in their probe of President Donald Trump’s alleged efforts to pressure Ukraine.

In an interview on CNN’s “State of the Union,” Schiff said the evidence against Trump is “already overwhelming,” although he stopped short of saying whether he would support impeachment himself.

“Yes, we’d love to have these witnesses come in,” Schiff said. “But we’re not willing to simply allow them to wait us out — to stall this proceeding — when the facts are already overwhelming.”

Democrats are seeking to prove Trump leveraged military assistance and an Oval Office meeting in exchange for investigations of former vice president Joe Biden and his son Hunter Biden and a debunked theory concerning purported Ukrainian interference in the 2016 presidential election.

Several key figures, including acting White House chief of staff Mick Mulvaney, Vice President Mike Pence, Secretary of State Mike Pompeo, former national security adviser John Bolton and Trump attorney Rudy Giuliani, have declined to cooperate with the impeachment inquiry.

A federal judge is expected to rule Monday on whether former White House counsel Donald McGahn must testify under subpoena.

Some have argued that Democrats should litigate the matter in the courts to force more witnesses to testify. But Schiff said Sunday that time is of the essence and that Democrats will continue to investigate even after they have submitted their report to the House Judiciary Committee.

“We’re going to continue our investigation. … The investigation isn’t going to end,” he said, adding that “we may have other depositions and hearings to do.”

He took particular aim at Bolton, arguing that the former national security adviser will have to explain why he chose to give his account of events “in a book” rather than show the “courage” that Fiona Hill, the former National Security Council Russia adviser, did in testifying before lawmakers last week.

Schiff declined to say how long it might take impeachment investigators to finish their report, saying only that “we’ll take the time that’s necessary.”

Trump, meanwhile, continued to take aim at Democrats, saying in a tweet on Sunday that they “are not getting important legislation done” because of the impeachment inquiry.

“USMCA, National Defense Authorization Act, Gun Safety, Prescription Drug Prices, & Infrastructure are dead in the water because of the Dems!” Trump said, referring to the U.S.-Mexico-Canada Agreement and other matters.

In another tweet, Trump claimed that public opinion has “turned very strongly against Impeachment, especially in swing states,” though national polls have shown that public sentiment about impeachment has remained stable.

According to an NPR-PBS-Marist poll, 49 percent of respondents supported removing Trump from office in mid-November. This is similar to the 48 percent who said the same in early October in another NPR-PBS-Marist poll.

Republicans on Sunday continued to defend Trump and sought to keep the focus on Biden, who is running for the 2020 Democratic presidential nomination.

Calling Trump’s alleged quid pro quo with Ukraine a “red herring,” Sen. John Neely Kennedy, R-Louisiana, said it’s important for lawmakers to understand why Trump asked Ukrainian President Volodymyr Zelensky for an investigation of the Bidens and what Hunter Biden did as a board member for Burisma, a Ukrainian energy company.

Kennedy said it was unfair that Trump was unable to call witnesses or offer a direct rebuttal during the initial proceedings of the impeachment inquiry.

“I think Speaker [Nancy] Pelosi’s judicial philosophy from the beginning has been ‘guilty,’” Kennedy said on “Fox News Sunday.” But he said that if there is a Senate trial, he does not think the allegations will be “summarily dismissed.”

“I’m in favor of doing it in accordance with due process and [letting[ everybody offer whatever they want to in terms of evidence,” Kennedy said, even if that “takes a long time.”

The Louisiana Republican said he was unsure whether Russia or Ukraine hacked the Democratic National Committee’s servers during the 2016 election, despite the intelligence community’s consensus that Russia was to blame. His comment drew a strong response from Rep. Eric Swalwell, D-California, who said, “It was Russia, and as a country, we have to make sure that we absolutely acknowledge it was Russia [and] condemn Russia for it.”

“It actually plays into Russia’s hands if they [Republicans] have this equivalence with Ukraine where we’re saying, ‘Well, maybe we don’t know which one it was,’” Swalwell said on “Fox News Sunday.”

In a separate interview on NBC News’ “Meet the Press,” Schiff said there is no longer a need for testimony from the anonymous whistleblower whose complaint is central to the impeachment inquiry — though Schiff had previously pledged that his panel would hear from the individual.

“We don’t need the whistleblower’s secondhand evidence anymore,” he said. “It would only serve to endanger this person and to gratify the president’s desire for retribution, and that is not a good enough reason to bring in the whistleblower.”

Schiff also pushed back against the argument that he himself should be called to testify in the Senate trial, noting, “I’m not a fact witness.”

“All I can relate is what the witnesses said in deposition and in the open hearings,” he said, arguing that calling him to testify would show a lack of seriousness by Senate Republicans.

On CNN, Schiff also declined to say whether he believes the House Ethics Committee should investigate Rep. Devin Nunes of California, the top Republican on the intelligence panel, over allegations that he met with an ex-Ukranian official to obtain information about Joe Biden and his son.

House Armed Services Committee Chairman Adam Smith, D-Washington, said Saturday that it was “quite likely” that Nunes would face a House inquiry. But Schiff said Sunday that he did not want to weigh in on the matter.

“I don’t want to comment on what the Ethics Committee should do, particularly vis-a-vis the ranking member of my committee,” Schiff said.

Washington Post writer Scott Clement contributed to this report.