Rep. Adam Schiff of California, the ranking Democrat on the House Intelligence Committee, said Sunday Chairman Devin Nunes has been a "serious obstacle" in the committee's investigation into possible collusion between President Trump's campaign and Russia.

"We continue the hard work of getting to the bottom of what happened. We faced very serious obstacles and many of them go back to our chairman, who, I think all too often, has been willing to further the work and the viewpoint of the White House, irrespective of what we are finding in the investigation. That's unhelpful," Schiff told ABC "This Week" host George Stephanopoulos.

"But nonetheless, there are plenty of Democrats and Republicans that continue to interview the key witnesses, review the documents and make progress in the investigation," Schiff added. "It's still my hope that notwithstanding all the turbulence created at the top of our committee that we can come to a common conclusion. But, it has been tough. I won't — I won't deny that for a minute."

Nunes announced in April he would recuse himself from the committee's probe in response to an ethics complaint that he improperly disclosed classified information. It's not clear if he is currently involved with the probe, though GOP Reps. Trey Gowdy of South Carolina, Mike Conaway of Texas, and Tom Rooney of Florida are supposed to be in charge of the investigation.

Schiff added the White House involvement in getting the Justice Department to lift a gag rule on an FBI informant on the investigation into a uranium deal done in 2009 was "unethical" and indicates deeper problems within the administration and Trump's supporters in Congress.

"If members of Congress are willing to essentially cover for that or worse, become complicit in that — and that was the word Jeff Flake used, in what I think is the most significant speech in Congress in the 17 years I have been there," Schiff said.

"If we allow ourselves in Congress to become complicit in unethical conduct by the president, in taking apart our system of checks and balances brick by brick, by intervening in the Justice Department with a gag rule, by interviewing U.S. attorneys in New York who may oversee potential prosecutions of the president's interests, by picking someone, confirming someone nominated to head the criminal division of the Justice Department who happens to be the lawyer for Alpha Bank, we become complicit in that kind of conduct, we will have to answer to history," Schiff added.

On Thursday, Schiff said he would investigate the Trump administration if he determined Trump "personally intervened w DOJ to advance" the case against his political opponent.