"There is circumstantial evidence of collusion", Schiff told Todd on "Meet the Press".
Nunes also said ahead of Monday's hearing he had seen "no evidence" of collusion between Russian Federation and Trump's team.
"I'll give you a very simple answer: No", he said.
This week he said neither Mr. Trump nor the White House would respond to further questions about the controversy until Congress finishes its investigation.
But Representative Jason Chaffetz, chairman of the House Committee on Oversight and Government Reform, said that he had yet to see any evidence of wiretapping.
Schiff was more circumspect when asked if Trump's taxes were necessary to the probe, telling reporters, "I think we have a lot of groundwork to do before we ever get to that question".
And last week, the Senate intelligence committee announced a March 30 hearing in connection to its investigation into Russian activities during last year's presidential election.
A second committee Democrat echoed Himes, calling the director of the Federal Bureau of Investigation and some former Obama officials also invited to testify an "easy ask" while saying that the Nunes-Schiff relationship will nearly certainly "be tested" over future requests.
Lindsey Graham, R-S.C., says he has a promise that Comey will appear before the Senate Judiciary Committee to answer two questions: did any USA court issue an order allowing the Trump campaign to be bugged and is there an open criminal investigation into Trump or his associates?
The issue mushroomed last month when Trump's national security adviser Michael Flynn resigned after it was revealed he misled top officials over his contacts with Russian Federation.
Chairman Devin Nunes, R-Calif., and ranking member Adam Schiff, D-Calif., originally requested the Justice Department send them copies of any wiretapping warrants, applications, court orders or other proof of Trump's allegations by this past Monday. "There's no evidence of collusion".More news: AARP to Report Health Care Votes
"We need to get to the bottom of that", Nunes said. "There's no little campfire".
Trump on March 4 tweeted that Obama had "tapped" his phone - a charge that has consumed political debate in the United States capital.
House Speaker Paul Ryan said he liked the chances of the American Health Care Act's passage at the scheduled House floor vote on Thursday.
The Justice Department on Friday delivered documents to congressional committees to help clear up whether the Obama administration spied on Trump.
The big question is whether Russian Federation interfered in the US election, as the USA intelligence community concluded in a January report.
The Department of Justice delivered documents to the House and Senate intelligence committees Friday regarding their request for information that could shed light on Trump's claim that Obama tapped his phones at Trump Tower in NY.
That runs counter to the president's assertion he was the unfair target of the Obama administration's surveillance - a charge both Democrats and Republicans have argued there is no evidence to back up. Thus far, the department has refused to do so.
Even White House press secretary Sean Spicer, who has been doing verbal somersaults in response to an avalanche of questions at his daily news briefings, seems to have thrown up his hands.
Trump said he and Merkel had "something in common", apparently referring to reports during the Obama administration that Merkel's phone was bugged. "And he needs to put an end to this". "As it relates to the Russians, we are happy to investigate it because I think Putin is a bad actor on the world stage".
One of those joint requests, signed last week, effectively launched the exclusively Republican demand that the committee's investigation include leaks of sensitive information to the media - an obvious point of compromise on the part of Democrats. "We think it's in the public interest that this be openly addressed by the director".