Midland MPs have attacked campaigning group 38 Degrees in the House of Commons - accusing it of intimidation and being hysterical.
Warley MP John Spellar claimed the organisation had a “nihilist, hysterical leadership” as he spoke in a Commons debate.
And the Labour MP also risked the anger of left-wing campaigners by defending a controversial planned trade treaty with the United States.
The treaty, called the Transatlantic Trade and Investment Partnership, has been described by pressure group War on Want as an attempt to roll back labour rights, food safety rules, regulations on the use of toxic chemicals and banking safeguards introduced to prevent a repeat of the 2008 financial crisis.
Left-wing journalist George Monbiot called it a “a reckless, unjustified destruction of our rights”, in a recent column for The Guardian.
Birmingham MP Roger Godsiff (Lab Hall Green) has also raised a series of concerns.
But Mr Spellar suggested critics were simply opposing the treaty because they didn’t like America.
Speaking in the Commons, he said: “Many of those campaigning on this issue are definitely not in favour of trade, and I have been on public platforms where they have actually declared as much. I think that they are also against capitalism, and they are definitely against anything to do with the United States.
“It is interesting to note that, whereas we have a huge number of trade agreements, this issue has only become contentious when the United States has become involved.”
And he specifically criticised 38 Degrees for campaigning against the treaty.
“We are talking about the creation of a free trade area and a trading and investing bloc amounting to about half the world’s GDP. That is significant to all of us who argue strongly that engagement in the European Union as part of a wider market is enormously important to working people in this country.
“Engagement in that much wider market, and, in particular, setting better benchmarks for world trade, is also enormously important, but it does not seem to have any impact on organisations such as 38 Degrees.
“Like other Members who have spoken, I have a slightly ambivalent attitude to 38 Degrees. It has some very decent supporters, many of whom are very concerned and engaged citizens, but it also has a nihilist, hysterical leadership.”
Mr Spellar said he agreed that the NHS should be exempted from the treaty, a key demand of some critics who say it could lead to US firms taking over NHS services.
38 Degrees has run a series of online campaigns, often focusing on what it calls privatisation of public services.
Not every MP appreciates the group, perhaps because it encourages members to send them mass emails.
Worcester Conservative MP Robin Walker also criticised the group in the Commons, attacking the behaviour of its Executive Director, David Babbs, when he gave evidence about the treaty to the Commons Business, Innovation and Skills Committee in November.
Mr Walker said, who is a member of the committee, said: “I am afraid that the executive of 38 Degrees has let its members down. The attitude that their representative arrived with appeared to be that parliamentary scrutiny - the very purpose of this motion and the object of our inquiry - was secondary to the campaign in which he was engaged, and that it was for MPs to passively accept the views that he was there to express and in no way to question them.”
He claimed the organisation “has launched an aggressive campaign of letter-writing and intimidation. A Labour colleague’s researcher has been reduced to tears, and a Conservative colleague who is always softly spoken and reasonable has been accused of being a harridan.”
38 Degrees was asked to provide a comment for this story but has not responded.