Leadership of the International Brotherhood of Teamsters has formally opened the door to the union poaching members of a fellow union, setting up the possibility of an internal fight within organized labor.

Teamsters President Sean OBrien sent a memo to the unions officers and organizers on May 23 informing them he had nullified their no-raid agreement with the International Association of Machinists and Aerospace Workers (IAM), according to a copy of the memo obtained by HuffPost.

No-raid agreements forbid unions from trying to organize one anothers members so that they defect to the other union. The AFL-CIO has a long-standing policy that bars raiding among its member groups, but the Teamsters are not part of the 60-union labor federation, only the IAM is.

The hard-charging OBrien has made a name for himself tussling publicly with corporate giants like UPS and Amazon as well as Republican lawmakers on Capitol Hill. He went viral last year after trading insults and nearly coming to blows with a GOP lawmaker in a Senate hearing.

The labor leader didnt hold back from criticizing a fellow union in his memo, accusing the IAM of crossing Teamster picket lines and lobbying for legislation that undermines our members in the trucking and delivery industries.

I informed the General President of the [IAM] that I was revoking any existing no-raid agreement and excluding the IAM and its local affiliates from the [Teamsters] general no-raid policy with respect to sister unions, OBrien wrote.

He said local Teamsters affiliates would not need permission to try to raid machinists members, but would have to notify headquarters immediately upon the outset of any such activity.

The IAM declined to discuss particulars about the spat. A spokesperson said in an email that the union looks forward to discussions with the IBT and continuing to keep the focus on growing the labor movement.

A Teamsters spokesperson said the union had nothing to add beyond what was said in the memo.

The rift comes at a time when organized labor writ large is trying to build on some high-profile victories and restore the union membership rate, which has fallen to just 6% in the U.S. private sector. Raiding can help a union grow, but it doesnt create more union members.