House Intelligence Committee Chairman Adam Schiff has been a common target for Republicans who believe that he is leading the impeachment inquiry against President Trump in an unfair way. House Republicans have decried the deposition of witnesses in closed hearings where only members of the three committees conducting the inquiry can attend, and Mr. Trump has often slammed "Shifty" Schiff on Twitter.

But in essence, Adam Schiff holds such a powerful position in overseeing the impeachment inquiry because the Trump Justice Department opted not to investigate a whistleblower complaint about a July 25 call between Mr. Trump and the Ukrainian president. As the Justice Department passed on an investigation, Democrats argue, it was up to Congress to investigate the call.

"The Justice Department was not going to get involved, therefore you're not going to have a kind of independent or special prosecutor. So then the House said, we need to prosecute this case ourselves, or at least begin the process. And that's what Republicans can't stand," said CBS News chief Washington correspondent Major Garrett said on CBSN on Friday. "They don't want Adam Schiff ... who they regard as a partisan leading the investigation."

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The two previous impeachment inquiries of the modern era involved either a special prosecutor or independent counsel.

"In Nixon you had a special prosecutor, in Clinton you had an independent counsel. Now you have a House Democrat leading the investigation," Garrett noted. That is a central difference between these three impeachment processes." He added, "But you can't eliminate the judgment and the intervention of the Trump Justice Department setting that in motion."

Democratic Congressman Jim McGovern, the chairman of the House Rules Committee, pointed out that the Intelligence Committee was taking the lead because of the Justice Department's inaction on the whistleblower complaint in an interview with WGBH on Thursday.

"We had a special prosecutor in the Clinton impeachment. (Independent Counsel) Ken Starr presented evidence to Congress, then the Judiciary Committee acted on it. This is different," McGovern said. "This Justice Department is not interested in investigating the president's behavior with regard to Ukraine. So it is left up to the Intelligence Committee. They've kind of acted as the special prosecutor."

On Thursday, the House passed a resolution defining the rules for the impeachment inquiry going forward, including in open hearings. The vote was 232 to 196, with two Democrats joining all Republicans in voting against passage.