Matthew Whitaker, the newly installed acting attorney general, built his career filing politically targeted lawsuits and complaints against Democratic lawmakers. In one case he prosecuted, a paid informant was instructed to wear a wire to an Alcoholics Anonymous meeting attended by a Democratic state senator.

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An examination of Whitaker’s record, and interviews with people who knew him in the early years of his career in Iowa, describe a lawyer who rose from obscurity to the top ranks of the Department of Justice (DoJ), where he is now the top law enforcement official in the country, because he was seen as a skilled partisan operator. Whitaker also received a critical helping hand over the years from a powerful patron: Chuck Grassley, the Republican senator from Iowa.

Whitaker’s relationship with Grassley, 85, who is chairman of the Senate judiciary committee and has direct oversight of the DoJ, raises significant questions about whether Grassley might seek to shield Whitaker despite concerns about his controversial – many say illegal – appointment by Donald Trump. It raises questions, for example, about whether Grassley would ever call a hearing in his oversight role in which Whitaker would have to testify under oath, opening him up to questions from Democrats.

“A fellow Iowan, who I’ve known for many years, Matt will work hard and make us proud,” Grassley said in a statement after Whitaker was appointed to replace Jeff Sessions following the forced resignation of the former Alabama senator. Last year, the senator posted an Instagram photo of the pair at breakfast.

Many legal scholars – both liberal and conservative – have said Whitaker’s promotion from Sessions’s chief of staff to acting attorney general is in violation of the appointments clause of the US constitution, which states that any administration official who reports directly to the president must be confirmed by the Senate.

Others, including the Democratic House leader, Nancy Pelosi, have called on Whitaker to recuse himself from overseeing the work of special counsel Robert Mueller, because of his previous criticism of the investigation. There are questions, too, about how close Whitaker has been to Trump while he served as Sessions’s chief of staff, with analysts suggesting he served as the administration’s “eyes and ears”.

Whitaker is also being scrutinised for remarks he has made about how judges should be Christian, and his work on the advisory board of a company that scammed military veterans and is reportedly under investigation by the FBI.

What has received less attention so far is the way the 49-year-old has persistently targeted Democrats over the course of his career, first in his work as a US attorney in Iowa from 2004 to 2009, and later as the head of a shadowy conservative activist group called the Foundation for Accountability and Civic Trust (Fact). It paid him hundreds of thousand of dollars in salary and is reportedly connected through a mystery company that has given it money to the conservative Koch brothers.

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Steve Roberts, a former head of the Iowa Republican party, told the Guardian Whitaker rose to prominence because of his willingness to run for statewide office against strong Democratic incumbents, which earned him a reputation as a “sacrificial lamb”. A star college football player who competed in the Rose Bowl, Whitaker caught the attention of Grassley, Roberts said. The senator campaigned across the state with Whitaker during his failed attempt to become state treasurer.

When a position opened as US attorney for the southern district of Iowa, Grassley recommended Whitaker to the Bush administration, despite Whitaker having scant legal experience. He wrote on a Senate questionnaire that his most relevant experience for the job was two breach of contract cases – one involving a dry cleaner, the other a home remodeling – and a personal injury case that was settled.

“Nobody gets appointed out here unless Grassley approves it,” Roberts told the Guardian. “It just doesn’t happen.”

Once he became US attorney, one of the most high-profile cases Whitaker’s office litigated was a 2007 trial against a popular Democratic state senator, Matthew McCoy, who is openly gay. McCoy was acquitted of charges that he extorted money from his business partner in the course of a dispute. In an interview with the Guardian, McCoy described how he believed Whitaker went to extraordinary and legally suspect lengths to target him.

The case Whitaker’s office brought against McCoy involved hours of wiretapped conversations collected by a paid informant who was discredited during the subsequent trial. The trial revealed that the informant had been directed to wear a wiretap to a meeting of Alcoholics Anonymous that McCoy had attended.

Facebook Twitter Pinterest Chuck Grassley, the Republican senator from Iowa and chairman of the Senate judiciary committee, gave critical helping hand to Whitaker. Photograph: Chip Somodevilla/Getty Images

“That was highly unprofessional,” McCoy said. “ I think because I was a rising star, because I was a popular Democrat, and because I was gay, they thought that I was an easy target and they were going to get me. And they did in a way, because it cost me a lot of money, about $100,000, and it cost me my reputation at the time. It hurt my family deeply.”

While McCoy’s legal team was repeatedly told by Whitaker’s team in discovery that the informant had never been paid, it was later revealed in court that he had. The presiding judge called the “lapse of memory” by Whitaker’s team “regrettable”.

McCoy told the Guardian he has had a successful career, but added that the shadow of the case hung over him, limiting opportunities to possibly run for Congress or governor.

“The point is they were defeated for being basically untrustworthy liars and cheats,” he said. “Matt Whitaker put his thumb on the scales of justice against me despite evidence that pointed to the contrary, and that tells you he is a very pliable person.”

The DoJ has defended the case in recent press reports, saying it was the result of a legitimate investigation that was not politically motivated.

As US attorney, Whitaker was also informed about a nine-month surveillance operation against anti-war protesters in Iowa in 2008 that some critics, including the American Civil Liberties Union (ACLU), later said possibly violated the rights of those who were surveilled. When the programme was discovered in 2010, Whitaker was in private practice and declined to comment to questions from the Des Moines Register about whether the operation had been “overkill”.

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When Barack Obama was elected, Whitaker left the US attorney’s office and unsuccessfully sought to run for various public offices.

In 2014 he joined Fact, an activist group that challenged Democratic candidates and supported Republican judicial causes, sometimes in ways that closely aligned with Grassley on the relevant committee. In one case, Fact urged the Federal Election Commission to investigate the Democratic National Committee and a consultant, claiming both broke federal law when the consultant, Alexandra Chalupa, allegedly met with Ukrainian officials seeking information about Trump during the 2016 campaign. Grassley also sought to prod the DoJ to investigate the matter. Chalupa has denied wrongdoing.

In another case, Whitaker called on the DoJ to investigate Sidney Blumenthal, the Democratic operative who is close to Hillary Clinton and has been a target of Grassley. Whitaker also sought an investigation into a Democratic candidate in California, Michael Eggman, but the FEC found no cause for action.

Although Fact’s tax status requires it to be non-partisan, a review by Newsweek found under Whitaker’s watch the organisation called for investigations into 46 individuals or organisations aligned with Democrats, and only a handful of Republicans.