Democrats on Sunday pushed back on Republican requests for testimony from the whistleblower who helped launch the impeachment inquiry against President Donald Trump, as well as former Vice President Joe Biden's son, Hunter Biden.

Rep. Devin Nunes, R-Calif., ranking member on the Intelligence Committee, requested that the whistleblower, the younger Biden and his business partner Devon Archer testify before House investigators in a letter Saturday to Rep. Adam Schiff, D-Calif., the committee's chairman. Later Saturday, Schiff poured cold water on the request, saying the impeachment probe would not serve "to carry out the same sham investigations into the Bidens or debunked conspiracies about 2016 U.S. election interference" that Trump asked Ukraine to conduct.

The whistleblower alleged that Trump tried to pressure Ukrainian President Volodymyr Zelenskiy in a July phone call to investigate the Biden family and conspiracy theory involving the 2016 election. Much of what the whistleblower alleged, based on second-hand information, has since been backed up by witness testimony, as well as a White House summary of the call. But Trump and his allies have sought to unmask the whistleblower, a CIA employee detailed to the White House, since his complaint was made public.

Through his legal team, the whistleblower has offered to provide written answers to the impeachment investigation.

"I think the Republicans are making an issue of anything that they think will give them some gravitas," Rep. Jackie Speier, D-Calif., told ABC's "This Week."

Speier, a member of the Intelligence Committee who has sat through the depositions of multiple Trump administration officials regarding Trump's conduct toward Ukraine, said those who had come forth to testify had provided "direct evidence" of Trump's efforts to pressure Ukraine, not "indirect evidence" that the whistleblower provided, which she said was "third-hand."

"We have Colonel [Alexander] Vindman, who was actually on the call, who will be in a position, I think, to testify," she said. "And so you have a much more direct person to speak to about the events. And you have the actual transcript that the president himself provided that is corroboration."

Sen. Amy Klobuchar, D-Minn., echoed Speier, saying on CNN's "State of the Union" that investigators "now have first-hand information of diplomats, military people, others that actually heard these actions occur, where the president was pushing for Ukraine to start an investigation against his political opponents in exchange for military assistance."

"That is what all of this mounting evidence is showing, and that's why I would agree with Adam Schiff," she said. "Why would you reveal the whistleblower when you are supposed to have protections ... for this whistleblower?"

Klobuchar, who is running for president, added that she sees "no reason" why Hunter Biden should be called to testify, saying Trump "was messing around to try to get information against a political opponent."

On NBC's "Meet the Press," Rep. Jim Himes, D-Conn., said Republicans were pushing a false equivalency between Trump and Biden's actions.

"The president of the United States demanding — extorting — a vulnerable country to do his political bidding, to go after his opponent, has nothing to do with Joe Biden executing the foreign policy of the United States,” Himes said of the former vice president's push, widely backed by the international community, to have Ukraine's top prosecutor dismissed for failing to investigate corruption.

"What the president did is wrong and impeachable," Himes said of Trump's push to have Ukraine probe Democrats and the Bidens.

Also on "Meet the Press," Sen. Rand Paul, R-Ky., argued that Trump's actions toward Ukraine were "exactly" the same as the former vice president's, an allegation that Himes pushed back on.

"This was American foreign policy, this was European Union policy, this was IMF policy that this prosecutor needed to go," he said.