WASHINGTON -- The wife of former Donald Trump campaign adviser George Papadopoulos was on Capitol Hill Wednesday speaking to Democrats on the House intelligence committee.

Simona Mangiante Papadopoulos was doing the interview with Democrats who are frustrated that the GOP-led House intelligence committee would not call more witnesses before it completed its Russia investigation earlier this year.

Papadopoulos pleaded guilty last year to lying to investigators about his contacts with people linked to Russia during the campaign. He then became a key co-operator for special counsel Robert Mueller, who is investigating Russian election interference and possible co-ordination with Trump's associates. Papadopoulos is expected to be sentenced in September.

His wife has been his frequent defender on Twitter and television, appearing on Fox News last month to lobby her husband's case.

"I trust and hope and ask to President Trump to pardon him," she said on Trump's favourite network.

The GOP-led intelligence panel's report issued this Spring said the probe found no evidence of "collusion, co-ordination or conspiracy" between the Trump campaign and the Russian government.

Democrats were furious, saying the Republicans had blocked many witnesses Democrats has wanted to call. Among those were Maria Butina, a 29-year-old gun-rights activist who was charged this week with being a covert Russian agent who gathered intelligence on American officials.

House Democratic Leader Nancy Pelosi briefly attended the beginning of the interview, where she said she welcomed Papadopoulos.

Pelosi said she told her "we seek the truth and we wish that the Republicans would have enabled this to happen in a bipartisan way."