 -- The top Democrat on the House Intelligence Committee, Rep. Adam Schiff of California, said on "This Week" Sunday that he doesn't "expect we're going to see any evidence" of President Trump's explosive wiretapping claims against his predecessor, adding that the unproven claim damages the president’s credibility.

President Trump claimed in a series of tweets last weekend that President Obama had ordered phones to be tapped at Trump Tower at the end of the 2016 campaign. The president has so far failed to provide any evidence to back up his claim, while calling on Congress to investigate the allegation. The leading members of the House Intelligence Committee in turn requested that the Justice Department respond by Monday with any evidence of the president’s claims.

“I don't expect we're going to see any evidence of this either,” Schiff told ABC News' chief anchor George Stephanopoulos Sunday. “There are one of two possibilities here. Either the president quite deliberately, for some reason, made up the charge. Or perhaps, more disturbing, the president really believes this.”

Either way, Schiff said the wiretapping claim has damaged Trump’s credibility and could have “real world consequences” in the future.

"If six months from now the president should say that Iran is cheating on the nuclear agreement, if he's making that up, it's real problem. If he's not making it up and it's true, it's an even bigger problem,” Schiff said. “Because the question is, would people believe him? Would American people believe him? Would people around the world believe him? And that has real world consequences."

Schiff said he believes the matter will be put to rest on March 20, when the House Intelligence Committee holds an open hearing, where FBI Director James Comey has been requested to testify.

"We're going to be able to ask the director of the FBI among others, is there any truth of this? Have they seen any evidence of this? They would be in a position to have to know. And I think on March 20th, if not before, we'll be able to put this to rest," he said. “But I don't think anyone really has any question about this, George. The only question is why the president would make up such a thing.”

Schiff has been critical of the FBI director in recent weeks, saying he had limited information to Congress on Russia's suspected meddling in the 2016 election process. But Schiff said Sunday he is "very pleased at the level of cooperation we're getting from the FBI.” Comey visited Capitol Hill Thursday to brief top members of the House and Senate.

“I have been very critical of the FBI and their willingness to cooperate in the investigation. I can say that that substantially changed,” Schiff said.

Schiff also said that he was “surprised” that former Director of National Intelligence James Clapper this week appeared to categorically deny there had been any collusion or cooperation between Trump associates and Russians, which the White House has denied.

“I wouldn't want to reach a conclusion at the outset of our investigation as to whether we're going to be able to demonstrate collusion,” Schiff said on “This Week.” “Maybe we will, and maybe we won't. Maybe it existed, and maybe it didn't. But I do think it's our obligation to do everything possible in as nonpartisan a basis as possible to answer those very important questions. And I wouldn't start out by concluding one way or the other.”

“We follow that evidence where it leads, and not leap to any conclusions on the front end,” Schiff added.

Schiff also responded to concerns from fellow Democrats that they will not be able to conduct a complete investigation, saying partisan politics could limit the investigation. “To be honest, George, I don't know whether we'll be able to conduct this to its completion, whether we'll have the kind of bipartisan cooperation that we need,” Schiff told Stephanopoulos.

“I think it's clear it's in the national interests to try,” Schiff added. “And if we get to the point where we can't, I think it's our obligation in the opposition to speak out and say this is not legitimate. We are being walled off from certain areas of investigation. But I think we owe it to the country to try, that's certainly my intention.”