The man accused of bludgeoning former US House of Representatives speaker Nancy Pelosi’s husband with a hammer was caught up in conspiracies when he broke into her home last year, his defence lawyer has said.

The attack on then-82-year-old Paul Pelosi in the early hours of October 28 2022 sent shockwaves through the political world just days before last year’s mid-term elections.

Defence lawyer Jodi Linker said she will not dispute that her client David DePape attacked Paul Pelosi, an encounter caught on police body camera video.

Instead, she will argue that he believed “with every ounce of his body” that he was taking action to stop corruption and the abuse of children by politicians and actors.

“This is not a ‘whodunit’. But what the government fails to acknowledge is the ‘whydunit’, and the why matters in this case,” she said.

DePape pleaded not guilty to attempted kidnapping of a federal official and assault on the immediate family member of a federal official with intent to retaliate against the official for performance of their duties. The crux of Ms Linker’s argument is that DePape is not guilty of the charges because he was not targeting Nancy Pelosi to try to stop her from engaging in her official duties.

Federal prosecutor Laura Vartain Horn told jurors that DePape started planning the attack in August and that the FBI evidence will show he researched his targets online, collecting phone numbers and addresses, even paying for a public records service to find information about Nancy Pelosi and others.

During her opening statement, Vartain Horn showed a photo of Paul Pelosi lying in a pool of blood. She also played a call DePape made to a television station repeating conspiracy theories.

“The evidence in this case is going show that when the defendant used this hammer to break into the Pelosis’ home he intended to kidnap Nancy Pelosi,” Vartain Horn said, holding a hammer inside a plastic evidence bag.

Prosecutors showed body camera video of DePape admitting to the attack while speaking to police and paramedics outside the Pelosis’ home.

“There is no denying what I did. Cops watched me do it,” he said.

Paul Pelosi is expected to give evidence next week.

DePape posted rants on a blog and an online forum about aliens, communists, religious minorities, and global elites. He questioned the results of the 2020 US election and echoed the baseless, QAnon conspiracy theory that claims the US government is run by a cabal of devil-worshipping paedophiles. The websites were taken down shortly after his arrest.

If convicted, DePape faces life in prison. He was also charged in state court with attempted murder, assault with a deadly weapon, elder abuse, residential burglary and other felonies. He pleaded not guilty to those charges. A state trial has not been scheduled.

In the courtroom on Thursday were Christine Pelosi, one of the Pelosis’ daughters, as well as Gypsy Taub, DePape’s ex-girlfriend, and Ms Taub’s and DePape’s two teenage sons. Ms Taub called DePape’s name softly and blew a kiss, and he smiled and waved in return.