One night in Washington last March, Devin Nunes received a call to head to the White House to see secret intelligence reports. Over the next week, he briefed the president, failed to keep his clandestine meeting a secret, spoke expansively – if confusingly – about classified information, and earned the ire and mockery of his colleagues in Congress.

Now Nunes is back. Confusion has followed.

One year after his spirited and lonely defense of Donald Trump’s claim that he was put under surveillance by the Obama administration, tweeted without evidence, the Republican lawmaker has rushed forward on his own to bolster the president’s criticisms of the FBI and justice department. He appears unchastened by a fellow Republican who said Nunes was running his own “Inspector Clouseau investigation”, undeterred by a brush with an ethics investigation, and unbothered by the many pronunciations of his Portuguese surname.

But on Thursday, his actions had so angered Democrats that the party’s leaders in the Senate and the House both called for his removal from the House intelligence committee.

Although the panel is charged with investigating Russian interference, it has split in recent months along partisan lines, with Republicans turning their scrutiny on the original FBI investigation into Russian meddling, which began in 2016. Nunes’s office has produced a controversial memo that reportedly suggests that the FBI acquired a wiretap on a Trump associate without telling a judge enough about their sources.

The memo links the FBI investigation back to a former British spy, Christopher Steele, whose research was paid for by Democrats, and who wrote a dossier on Trump that contains a series of controversial, though unverified, claims. Republicans on the committee have also fixated on texts criticizing Trump by an FBI agent, Peter Strzok, who was temporarily in charge of the separate investigations into Hillary Clinton and Trump. Strzok also pushed for reopening an investigation into Clinton before the 2016 election, and was removed from the Trump investigation by special counsel Robert Mueller.

Q&A What is the Nunes memo? Show The memo was written by aides to Devin Nunes, chairman of the House intelligence committee and a member of the Trump transition team. The committee is investigating Russian interference in the 2016 election but the inquiry has devolved into a fight about the ​separate FBI investigation​, now​ led by special counsel Robert Mueller​. On Friday, Nunes published the memo after Donald Trump declassified it. The memo revolves around a wiretap on Carter Page, an adviser to the Trump campaign, alleging the FBI omitted key information when it applied for the wiretap. The findings “raise concerns with the legitimacy and legality of certain DoJ and FBI interactions” with the court that approves surveillance requests, the memo says. It also claims “a troubling breakdown of legal processes established to protect the American people from abuses”. The memo criticizes investigators who applied for the wiretap, saying they used material provided by an ex-British agent, Christopher Steele, without sufficiently disclosing their source. The memo says Steele was “desperate that Trump not get elected”. The memo also says texts between an FBI agent and FBI attorney “demonstrated a clear bias against Trump” and says there is “no evidence of any co-operation or conspiracy between Page” and another Trump aide under investigation, George Papadopoulos. The memo casts deputy attorney general Rod Rosenstein in a negative light. ​Rosenstein could fire Mueller. The president, said to dislike Rosenstein, could fire and replace him. The FBI ​argued against the memo’s release. Democrats wrote a rebuttal and sided with the bureau. ​The president reportedly told associates he believes the memo will help discredit the special counsel. Alan Yuhas

The FBI has urged against the memo’s release, suggesting its contents were misleading, in a rare unsigned statement: “We have grave concerns about material omissions of fact that fundamentally impact the memo’s accuracy.” The texts were revealed in the course of an internal justice department review into how the FBI handled an investigation of Clinton’s private email servers.

Both the memo and the texts dovetail with allegations of FBI bias propagated on rightwing media, such as Fox News, which the president watches frequently. Hosts such as Sean Hannity, who is close to Trump, have used them as a stick to batter the integrity of the FBI investigation into Russian meddling and potential obstruction of justice by the Trump White House.

Nunes, who has provided much of this fodder, was one of Trump’s first supporters in Congress and an adviser to his transition team after the election. Despite his stated outrage about a wiretap on one of Trump’s former campaign advisers, he has strongly supported government surveillance powers since he was elected in 2002 to Congress, aged 29, by a rural district of central California. He began his career in public service there six years earlier, as a board member of a community college.

Most recently, Nunes worked hard to extend the government’s surveillance powers for six years, in some cases without a warrant. Trump signed the bill into law last month, after writing a series of contradictory tweets. Nunes’s efforts have not gone entirely unnoticed. Asked whether he felt vindicated by the lawmaker’s sudden visit to the White House to give a briefing based on White House sources, Trump told reporters, “I somewhat do.”

“I very much appreciated the fact they found what they found,” he said.