(CNN) - Rep. Adam Schiff, the ranking Democrat on the House Intelligence Committee, said Thursday that Republicans are blocking dozens of witnesses from being interviewed in the Russia probe, including some who were aware of the 2016 Trump Tower meeting where Donald Trump Jr. was promised dirt on the Clinton campaign.
In a briefing with reporters, Schiff accused Republicans of blocking the inquiry from moving ahead, leaving key questions unanswered. He said that the House Intelligence Committee has spoken to 56 witnesses, fewer than half of the number in the Senate Intelligence Committee. Among the witnesses he wants to interview: Ivanka Trump, plus he says the panel should consider issuing a subpoena to Trump Jr. for information he withheld during his classified hearing last year.
"We can't do a thorough and credible investigation unless we obtain these documents -- unless we talk to these witnesses," Schiff said.
Schiff placed the blame largely on Republican Devin Nunes, saying the GOP chairman of the committee -- who stepped aside from running the investigation for much of last year -- has blocked dozens of requests for witnesses and subpoenas, with the support of House Speaker Paul Ryan.
The White House declined to comment on Schiff's accusations. Aides to Nunes and GOP Rep. Mike Conaway, who is running the Russia probe, did not respond to requests for comment. An attorney for Trump Jr couldn't be reached for comment, and an Ivanka Trump attorney had no immediate comment.
"While Mr. Schiff tries to distract from the serious, bipartisan review that's been underway for nearly a year, we will stay focused on following the facts and working to safeguard the upcoming election," said Ryan spokeswoman AshLee Strong.
The assessment from the committee's top Democrat underscores the partisan impasse that has plagued the House investigation, with Nunes and House Republicans now focusing on investigating allegations of improper FBI conduct during the 2016 elections and over its Russia probe -- while Democrats demand a deeper dive into whether there was any Trump campaign collusion with the Russians.
Among the questions Schiff says the probe has not answered: whether the former Trump foreign policy adviser George Papadopoulos -- who pleaded guilty to lying to the FBI about his contacts over Russia -- had communications with the Trump campaign about those efforts. Plus, Schiff says there has been scant interest in probing efforts by the late GOP operative Peter Smith, who sought the hacked Hillary Clinton emails during the election year. And Schiff says the committee needs to press Attorney General Jeff Sessions over key questions he did not answer, including whether the President took steps to hinder the Russia investigation.
Moreover, Schiff said the GOP has blocked Democrats' demands to request documents and hold witness interviews about the misleading responses to the news media after the Trump Tower meeting was revealed last summer. And he says the GOP has not supported efforts to get information from the White House about the firing of former FBI director James Comey, saying the White House has ignored bipartisan requests while the GOP has done little to pursue it. Plus, he argued, the House panel needs to subpoena Deutsche Bank for the financial records of the Trump Organization to learn about any financial arrangements with Russian interests.
"This investigation has gone about 1/4 of the time of the Benghazi investigations," Schiff said, referring to the GOP-led inquiries after the 2012 terrorist attacks in Benghazi, Libya. "But part of the reason why this investigation has taken longer than it should is because the majority has sat on our requests for witnesses for months and months and months."
The GOP has agreed to bring in a handful of witnesses so far this year, including former senior Trump aide Steve Bannon, who is expected before the panel next week. But Schiff says that's not enough.
Among the major topics still of interest to Schiff: the 2016 Trump Tower meeting with Russian operatives that occurred after Trump Jr. was told he would get dirt on the Clinton campaign and that the Russian government wanted to see his father win the presidency.
Even though the committee has interviewed nearly all the participants in the Trump Tower meeting, Schiff says there are additional witnesses "who knew the subject" of the Trump Tower meeting that need to be interviewed. He declined to say whom these individuals were when asked.
"It may be necessary for us to (release the names) in future if the majority does shut down the investigation or put it in some kind of hibernation," Schiff warned. "There are witnesses who have knowledge of the Trump Tower meeting and what it was about who have not been brought to our committee -- and need to."
Moreover, Schiff contended that Trump Jr. needs to provide more information after he declined to reveal to the panel the contents of a conversation he had with his father in 2016 after the Trump Tower meeting was initially revealed in The New York Times. Trump's son invoked attorney-client privilege, telling the House panel that he did not want to reveal the contents of their conversation because it was in presence of their lawyers, according to Schiff. Schiff says the GOP should agree to subpoena for that information if Trump Jr. does not provide details to the committee.
"Without getting complete phone records .. we will not get a full picture of what conversations may have taken place between the president and his son contemporaneous with the Trump Tower meeting," Schiff said.
Schiff said one of Trump's daughters -- Ivanka -- could provide valuable information to the investigation, but he said the GOP has yet to back efforts to request an interview with her.
"I think it would be valuable for her to testify and come before the committee," Schiff said.