The Anti-Defamation League has sent the House speaker, Paul Ryan, a letter requesting that Steve King, the controversial Iowa Republican, be stripped of committee assignments and formally censured for what the organization said was a “disturbing series of involvements and statements … that are antisemitic and offensive not just to the Jewish community, but to all Americans”.

In its letter, the ADL pointed out that King had relationships with Austria’s far-right Freedom party, which has stumbled through repeated neo-Nazi scandals; that King had deployed an antisemitic smear about George Soros, the Jewish-Hungarian philanthropist; and that King had endorsed foreign politicians linked to antisemitic and neo-Nazi groups.

The letter was the latest in a series of extraordinary setbacks for King, an eight-term congressman who has regularly been re-elected in Iowa’s fourth congressional district by double-digit margins.

The national Republican party withdrew support from King on Tuesday, with a tweet from Representative Steve Stivers of Ohio, chairman of the National Republican Congressional Committee, which is in charge of defending the Republican House majority.

“Congressman Steve King’s recent comments, actions, and retweets are completely inappropriate,” Stivers tweeted . “We must stand up against white supremacy and hate in all forms, and I strongly condemn this behavior.”

In response to a request for comment on Stivers’ tweet, AshLee Strong, a spokesperson for Ryan directed the Guardian to a statement the House speaker had issued in June, when King retweeted a well-known British neo-Nazi: “The speaker has said many times that Nazis have no place in our politics, and clearly members should not engage with anyone promoting hate.”

Also on Tuesday, Land O’Lakes, the dairy and animal feed company with large agricultural interests in King’s district, announced it would no longer support the congressman. “Representative King’s recent statements are in conflict with our values and we are no longer contributing to his campaign,” the company said.

On usually solidly Republican turf, King also appears to suddenly have a close race on his hands against his challenger, the Democrat JD Scholten. A reputable poll published on Tuesday had Scholten only one point behind King ? in a district that Donald Trump won by 27 points.

King invoked Trump in a statement defending himself from the tidal wave of criticism. “These attacks are orchestrated by nasty, desperate, and dishonest fake news,” King said. “Their ultimate goal is to flip the House and impeach Donald Trump. Establishment Never-Trumpers are complicit.”

ADL’s director, Jonathan Greenblatt, wrote that the letter was prompted by the synagogue shooting in Pittsburgh at the weekend in which 11 were killed. “After the events of this weekend, I knew that ADL could be silent no more,” he wrote.

“Rep King has brought dishonor on to the House of Representatives ,” Greenblatt added. “We strongly urge you and the congressional leadership to demonstrate your revulsion with Rep King’s actions by stripping him of his subcommittee chairmanship and initiating proceedings to formally censure or otherwise discipline him.”

In a campaign stop on behalf of local Republicans including King in Iowa earlier this month, Trump approvingly said King “may be the world’s most conservative human being”.