President Trump joined congressional Republicans in publicly rejecting the Ukraine call whistleblower's offer to respond to written questions from Republican lawmakers, instead insisting that he or she appear in person as part of the impeachment inquiry -- a move that would reveal the anonymous official's identity.

The whistleblower's attorney, Mark Zaid, tweeted over the weekend that his client -- whose complaint about Trump's July phone call with the president of Ukraine touched off the impeachment probe -- would provide sworn, written answers under penalty of perjury. Trump in turn attacked the whistleblower's credibility and demanded in-person testimony.

REP. JIM JORDAN REJECTS WHISTLEBLOWER'S OFFER TO PROVIDE WRITTEN ANSWERS TO GOP QUESTIONS

"The Whistleblower gave false information & dealt with corrupt politician Schiff. He must be brought forward to testify," the president tweeted Monday morning. "Written answers not acceptable! Where is the 2nd Whistleblower? He disappeared after I released the transcript. Does he even exist? Where is the informant? Con!"

Trump was referring to the whistleblower's early interaction with Schiff's staff. The whistleblower's central allegation that Trump in July urged Ukraine to launch politically related investigations, however, has been supported by other witnesses as well as the call transcript released by the White House.

But the president joins House Oversight Committee ranking member Rep. Jim Jordan, R-Ohio, in calling for the whistleblower to come forward and testify in person.

Late Sunday, Jordan seemingly rejected Zaid's offer, saying, "written answers will not provide a sufficient opportunity to probe all the relevant facts and cross examine the so-called whistleblower."

Jordan has claimed the only one in Congress who knows the whistleblower's identity is House Intelligence Committee Chairman Rep. Adam Schiff, D-Calif., who is leading the impeachment inquiry. Zaid insists that Schiff has not personally had any contact with the whistleblower or their legal team.

The Republican allegations about Schiff's ties to the whistleblower stem from the revelation that in the weeks after Trump's July 25 phone call with Ukrainian President Vlolodymyr Zelensky and the filing of the whistleblower's complaint, the whistleblower was in touch with Schiff's staff but failed to disclose this.

Trump has used this as a talking point for casting doubt on the complaint and the ensuing impeachment inquiry that is currently exploring whether the president pressured Ukraine into investigating his political opponents.

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Current and former Trump administration officials have testified regarding the phone call and surrounding events, as Democrat-led House committees attempt to learn more about the context of Trump's request for Zelensky to assist in the investigations of Democratic activities during the 2016 election as well as former vice president Joe Biden and his son Hunter, specifically pertaining to Hunter's business dealings with Ukrainian energy company Burisma Holdings.

Zaid tweeted Sunday that he and his team have "directly engaged GOP as to the irrelevance of the whistleblower's information and identity."

Fox News' Gregg Re, Chad Pergram and The Associated Press contributed to this report.