(CNN) Democrats are ratcheting up pressure on House Speaker Paul Ryan to intervene in the growing controversy involving House Intelligence Committee Chairman Devin Nunes, who quietly changed his explosive memo alleging FBI abuse without informing many of his colleagues.

The top Democrats in the Senate and the House say Republicans have "decided to sow conspiracy theories" and "attack the integrity and credibility of federal law enforcement as a means" to protect President Donald Trump and undercut special counsel Robert Mueller.

Senate Minority Leader Chuck Schumer sent a letter Thursday to Ryan, obtained by CNN, with a long list of questions ranging from the FBI and Justice Department objections to the letter to whether Ryan's staff was involved in drafting the memo and if the edits to the document were consistent with House rules.

"Quite simply, under your leadership, dangerous partisanship among many House Republicans seems to have taken precedent over the oath we all take to protect our nation," Schumer wrote.

House Minority Leader Nancy Pelosi also sent a letter to Ryan calling Nunes' actions "dangerous" and "illegitimate," and called on Ryan to remove Nunes as Intelligence Committee chairman.

"It is long overdue that you, as Speaker, put an end to this charade and hold Congressman Nunes and all Congressional Republicans accountable to the oath they have taken to support and defend the Constitution, and protect the American people," Pelosi wrote. "The integrity of the House is at stake."

House Speaker Paul Ryan responded in a news conference Thursday by defending House Intelligence Chairman Devin Nunes and supporting the release of the committee's Republican memo that alleges abuses of the FISA surveillance law. He said the Nunes memo "does not impugn the Mueller investigation or the deputy attorney general," Rod Rosenstein.

"Congress doing its job in conducting legitimate oversight over a very unique law, FISA, and if mistakes were made and if individuals did something wrong then it is our job as the legislative branch of government to conduct oversight over the executive branch if abuses were made," Ryan said. "What this is not is an indictment on our institutions of our justice system. This memo is not an indictment of the FBI, of the Department of Justice."

Ryan dismissed the calls to remove Nunes as chairman, saying Democrats were "playing politics."

The four-page memo spearheaded by Nunes and his committee staff alleges that the FBI and Justice Department abused the Foreign Intelligence Surveillance Act by using the opposition research dossier on Trump and Russia to obtain a surveillance warrant on former Trump campaign foreign policy adviser Carter Page.

House Republicans say the memo reveals abuses at the senior levels of the FBI and Justice Department, and the House Intelligence Committee Republicans voted Monday to make the classified memo public.

But Democrats have charged that the memo is misleading and inaccurate and is an effort to undermine Mueller's probe.

The memo is now in the hands of the White House, which under the committee's rules has five days to decide after it is received whether the memo should be public or object to its release. Trump has signaled he's inclined to release it, and he's telling associates he believes it could discredit the Russia investigation.

A senior administration official told reporters Thursday the White House will tell Congress "probably tomorrow" that the President is OK with the Nunes memo, and the official doubted there would be any redactions.

The FBI and Justice Department, however, have objected to its release, and the FBI issued a statement Wednesday saying the bureau had "grave concerns about material omissions of fact that fundamentally impact the memo's accuracy."

A new controversy over the memo broke out late Wednesday when the Intelligence Committee's top Democrat, Adam Schiff of California, said Nunes had altered the memo that was sent to the White House from what the committee voted to approve.

A senior Democratic committee official told CNN that there are "five material changes to different parts of the memo, including a modification that appears intended to water down the overall importance of the majority's purported 'findings.'"

"The majority document now in the White House's possession is different in key respects from the document the committee majority voted on January 18, 2018 to make available to the full House of Representatives and which hundreds of House members have read so far," the official said.

A Nunes spokesman confirmed changes had been made Wednesday, but argued they were minor and at the request of the FBI and Democrats.

Republicans on the committee downplayed the Democratic concerns about the alterations.

"Rep. (Trey) Gowdy is not concerned," said Amanda Gonzalez, Gowdy spokeswoman. "The edits were implemented before House Intel voted to make the memo public, so there is no need to vote yet again. The edits were also minor technical changes. Two edits were specifically requested by the FBI and the minority."

Ryan also disputed Democratic calls for the committee to revote on making the memo public.

"The process is exactly what it should have been," Ryan said.

Well before the committee voted to make the Nunes memo public, Ryan has aided the California Republican in his months-long effort to gain documents from the FBI and the Justice Department about the dossier that were used to draft the memo.

"I think disclosure is the way to go. It's the best disinfectant," Ryan said Tuesday after the committee's vote. "And I think we need to disclose, that brings us accountability, that brings us transparency, that helps us clean up any problem we have with DOJ and FBI."