Two top think tanks in Washington are mulling whether to sever ties with a controversial former AIPAC spokesman after it emerged that he was encouraging conservative writers to echo charges that critics of Israel are guilty of anti-Semitism, according to sources familiar with the discussions.

The fate of the former AIPAC spokesman, Josh Block, will be a big deal to people in left-leaning foreign policy circles in Washington. For them, the question of whether the think tanks will remain affiliated with Block will be seen as a referendum on the larger issue of whether demeaning Israel critics as anti-Semitic will be considered acceptable discourse among foreign policy experts.

Block stirred a lot of discussion and anger among foreign policy types when Salon reported last week that he was shopping “opposition research” on bloggers critical of Israel to friendly neoconservative journalists. Salon said that Block threw around accusation of anti-Semitism on a friendly listserv, calling on others to “echo” and “amplify” his efforts to “attack the bad guys.” The Salon story came after Block accused writers at the left-leaning Center for American Progress of writing “borderline anti-semitic stuff.”

Now the Progressive Policy Institute and the Truman National Security Project are privately considering a formal break with Block, who is a senior fellow at PPI and a member of Truman, sources involved in the discussions say.

PPI head Will Marshall privately told Block that the think tank would sever ties with Block if he didn’t retract the charges detailed in Salon, according to a source familiar with the discussions. Block subsequently offered Politico a statement on the charges, claiming he had never accused people at CAP in particular of anti-Semitism, but not walking back or apologizing for the gist of what was reported in the Salon piece. It’s still unclear how PPI — which declined to comment — will proceed at this point.

Meanwhile, at Truman, top officials privately debated via email whether to cut ties with Block after the Salon story broke, a source says. They had already been unhappy with Block’s attacks on critics of Israel, and the Salon piece exacerbated tensions, I’m told.

“Personal attacks have no place in our community,” Truman spokesman Dave Solimini tells me. “That agreement is unbreakable. The trust built among members of the truman community is the issue here. Personal attacks on members of our community, like calling them anti-Semitic, would cross that line.”

Separately, Lanny Davis, a business partner of Block, has condemned his attacks on CAP, claming that impugning anti-Semitic motives to the Center’s writers is “wrong.”

The dust up offers a glimpse of an insular and largely unseen world that has a vast impact on the debate in Washington, a place where plum think tank gigs are key to maintaining an aura of influence. It also sheds light on how intense the battle over what it means to be “pro Israel” has become, now that left leaning groups are mounting a serious challenge to the reigning and long unchallenged Washington consensus about what that term means.

UPDATE: One CAP blogger has apologized for using the term “Israel-firster,” saying he wasn’t aware of the “connotations” of the term, but it’s unclear whether this will change the calculus of the think tanks evaluating Block’s tenure.