Washington (AFP) - A Republican lawmaker on Sunday broke with the US president and fellow party members to reject the idea that the whistleblower whose complaint prompted an impeachment inquiry into Donald Trump should have to testify publicly.

After weeks of closed-door hearings, House committees on Wednesday will begin questioning witnesses in open session about efforts by Trump and his aides to pressure Ukraine to investigate political opponents including former vice president Joe Biden and his son Hunter.

Republicans on the House intelligence committee have asked its chairman, Adam Schiff, to call as witnesses both Hunter Biden, who had business links to Ukraine, and the still unidentified whistleblower, who is known to work in the intelligence community.

Republican congressman Will Hurd, himself a former CIA officer, spoke out Sunday against including the whistleblower -- the target of angry attacks by Trump and many of his supporters.

"I think we should be protecting the identity of the whistleblower," Hurd said on "Fox News Sunday."

"How we treat this whistleblower will impact whistleblowers in the future. Having this whistleblower law on the books is important."

A 1989 law protects federal government employees who report possibly illegal activity, gross mismanagement, abuse or waste of funds. The law bans any retaliation against such persons.

Trump has repeatedly denounced the whistleblower, saying he or she had only second-hand information and suggesting that whoever gave that person sensitive information was "close to a spy" and should face severe consequences.

Schiff on Saturday accused Republicans of pursuing "sham investigations into the Bidens" and said he would not "facilitate the president's effort to threaten, intimidate and retaliate against the whistleblower, who courageously raised the initial alarm."

Another Democratic member of the committee, Jim Himes, said Sunday that when people hear from respected officials with first-hand knowledge of the pressure campaign against Ukraine "the president's defense... which is all these people had second-hand knowledge, that's going to crumble."

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And Senator Amy Klobuchar, who is seeking the Democratic presidential nomination, told CNN that "all sources tell us that what the whistleblower said was second-hand information, and they now have first-hand information of diplomats, military people, others that actually heard these actions...

"Why would you reveal the whistleblower?"

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