House Republicans plan to call Intelligence Committee Chairman Adam Schiff as one of their first witnesses in the impeachment inquiry against President Trump following the adoption of formal rules for the investigation, claiming he is a “fact witness” due to his office's early involvement with the whistleblower whose complaint sparked the proceedings.

A source familiar with Republicans’ strategy moving forward in the impeachment inquiry confirmed to Fox News on Monday that GOP members plan to call Schiff, D-Calif., for questioning -- even if they are unlikely to succeed.

TRUMP SAYS UKRAINE WHISTLEBLOWER 'MUST' TESTIFY, BLASTS OFFER OF WRITTEN ANSWERS

The source told Fox News that Republicans want answers to questions like: “How many times did he meet with the whistleblower? What did they advise the whistleblower to do? How much was Schiff involved in this? Did he recommend the whistleblower give the complaint to the intelligence community inspector general, even though there was no intel component, so that he could be involved?”

Schiff maintains that he has not personally spoken with the anonymous whistleblower. However, it was revealed several weeks ago that the whistleblower at least had early contact with his office, essentially giving them a heads-up about the complaint concerning Trump's July phone call with Ukraine's president.

Regarding that early discussion, the GOP source noted that Republicans could be interested in hearing from the “anonymous” Schiff staffer involved.

Republicans' effort to devise a strategy going forward comes after the House approved rules for the process last week. While Republicans opposed the resolution and complained the rules were unfair, they still gave minority Republicans the ability to subpoena witnesses, with the concurrence of Democratic committee chairs. If the chair does not consent, the minority can appeal to the full committee.

This process still gives Democrats final say over witnesses, however, and the GOP source acknowledged it's unlikely they would go along with the efforts to call Schiff -- who is essentially leading the impeachment probe.

But GOP lawmakers for days had telegraphed that they were interested in making the attempt.

House GOP Leader Kevin McCarthy, R-Calif., said Sunday on CBS' "Face the Nation" that Schiff is the "first person" who should be brought in, along with his staff.

REPUBLICANS SLAM PELOSI OVER IMPEACHMENT REVERSAL AFTER FLOOR VOTE: 'WHAT HAS CHANGED?'

Last week, House Judiciary Committee Ranking Member Doug Collins, R-Ga., publicly challenged Schiff to come before the judiciary panel.

“Come to the Judiciary Committee," Collins said after the passage of the impeachment rules resolution. "Be the first witness and take every question asked of you. Starting with your own involvement of the whistleblower.”

Schiff’s office last month said that the whistleblower had reached out to them before filing the complaint in mid-August, giving Democrats advance warning of the accusations that would lead them to launch an impeachment inquiry days later. The inspector general complaint about Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky flagged concerns about efforts to pressure Ukraine to investigate former Vice President Joe Biden and his son Hunter as military aid to the country was being withheld.

A transcript released by the White House shows Trump making that request, but he and his congressional allies deny, and plan to continue to deny, that military aid was clearly linked to the request, or that there was any "quid pro quo." Some witnesses coming before House committees as part of the impeachment proceedings have challenged that assertion.

Meanwhile, Republicans are also hoping to call the whistleblower to testify, according to the source, who pointed to Schiff’s recent reversal on the issue.

Schiff in September had previewed testimony from the whistleblower “very soon,” but in recent weeks has suggested that testimony is unnecessary.

The president, repeatedly, has called for the individual to testify.

"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!"

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.

The whistleblower's attorney, Mark Zaid, tweeted over the weekend that his client would provide sworn, written answers under penalty of perjury.

But late Sunday, House Oversight Committee Ranking Member Jim 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."

Republicans also plan to continue to criticize House Speaker Nancy Pelosi for not holding a formal floor vote on the impeachment inquiry process until a month after announcing the probe, and for crafting rules they say limit their ability to subpoena witnesses.

According to another GOP source familiar with the impeachment process, Republicans plan to continue arguing that the entire impeachment inquiry against Trump is a “sham,” and push back against the substance of the inquiry itself.