Senator Mark Warner said on Wednesday that a meeting he had scheduled at the headquarters of a US intelligence agency was cancelled following online attacks by the far-right activist and conspiracy theorist Laura Loomer.

Warner, the Democratic vice-chair of the Senate intelligence committee, was set to visit the National Geospatial-Intelligence Agency in Virginia in what he described part of his “responsibility to provide oversight and support to our intelligence community”.

The administration rescinded the invitation after Loomer initiated a “campaign of baseless attacks” against him and the agency’s director, Trey Whitworth, he said.

“I can’t overstate how unprecedented and dangerous this is,” Warner said in a fundraising email. “This administration is taking its marching orders from Laura Loomer – a wackjob with a long history of outlandish fringe views, including 9/11 denialism, anti-Muslim harassment campaigns, and associations with white supremacists.”

Loomer posted on social media in recent days complaining that the director of an intelligence agency was hosting a “rabid ANTI-TRUMP DEMOCRAT SENATOR”. She celebrated the cancellation, calling Warner a threat to national security and arguing he should be removed from the Senate committee.

“He weaponized our intelligence agencies to push the debunked Russia Collusion Hoax,” she wrote.

She told the New York Times Warner should “be removed from office and tried for treason”.

Warner told reporters that the decision to cancel the previously unpublicized meeting was made by the office of the defense secretary.

The incident illustrates Loomer’s enduring influence within Donald Trump’s administration. The 32-year-old, who has previously described herself as “a proud Islamophobe”, has acted as a national security and foreign policy adviser to the president. In April, Trump fired six staffers after Loomer gave him a list of people she believed were not sufficiently loyal to the president.

Last month, the administration announced it was planning to stop issuing visas to children from Gaza seeking medical care after complaints from Loomer.

Warner argued that Loomer is “basically a cabinet member at this point” and that Trump and his administration were “caving to whatever she wants”.

“This nakedly political decision undermines the dedicated, nonpartisan staff at [the] NGA and threatens the principle of civilian oversight that protects our national security,” Warner said in a statement.

“Members of Congress routinely conduct meetings and on-site engagements with federal employees in their states and districts; blocking and setting arbitrary conditions on these sessions sets a dangerous precedent, calling into question whether oversight is now allowed only when it pleases the far-right fringe.”

Authorities in Argentina have recovered an 18th-century painting stolen more than 80 years ago by the Nazis from a Jewish art dealer in Amsterdam, a week after it was spotted by chance in a real estate listing.

The painting, the long-lost Portrait of a Lady (Contessa Colleoni) by the Italian master Giuseppe Ghislandi, was looted in the second world war. It was handed over on Wednesday to the Argentinian judiciary by the daughter of the late Nazi financier Friedrich Kadgien, Patricia Kadgien, who has been under house arrest with her husband since Tuesday.

Prosecutors allege the couple tried to conceal the stolen artwork. They face a hearing on Thursday on charges of concealment and obstruction of justice. The Guardian contacted her legal representatives, who declined to comment.

The Dutch newspaper AD traced the painting after a years-long investigation that took a breakthrough turn last week when one of its reporters found Kadgien’s house in an online property listing in the seaside city of Mar del Plata.

A photo in the listing showed the missing artwork – last seen in 1946 and belonging to the Dutch Jewish art dealer Jacques Goudstikker – hanging above a sofa in the couple’s living room. AD published its findings on 25 August.

The next day, federal prosecutor Carlos Martínez ordered a raid on the property, but the painting was no longer there. Police seized two unlicensed firearms and two mobile phones.

Four additional raids on Monday uncovered two other paintings that experts believe could date back to the 19th century, along with several drawings and engravings. The judiciary is analysing the works to determine whether they, too, were looted during the second world war.

A federal court in Mar del Plata placed Kadgien and her husband under 72-hour house arrest on Tuesday.

After the fall of the Third Reich at the end of the second world war, several high-ranking Nazi officials fled to South America.

Friedrich Kadgien was among them. He fled the Netherlands in 1946, first to Switzerland, then Brazil, and finally to Argentina, where he had two daughters. The painting is believed to have accompanied him and to have remained in his family’s possession after he died in Buenos Aires in 1978.

The portrait was among more than 1,000 works of art stolen by the Nazis from Goudstikker, who died in 1940 after falling in the hold of the ship carrying him to safety.

Goudstikker’s heirs plan to reclaim the painting, AD reported.