Harvard Law professor and pro-Israel advocate Alan Dershowitz has threatened to leave the Democratic Party if it elects Rep. Keith Ellison (D-Minn.) as its chair this weekend, amid controversy surrounding the congressman’s past actions and statements on Israel and anti-Semitism.

“My loyalty to my country and my principles and my heritage exceeds any loyalty to my party,” Dershowitz, who supported Hillary Clinton in 2016, wrote in an op-ed for The Hill on Friday.

Dershowitz cited what he called Ellison’s “long history of sordid association with anti-Semitism” including his work with notorious anti-Semite Louis Farrakhan, of the Nation of Islam movement. Ellison has admitted working with the group while in law school but insists he was never a member and has since denounced it as “bigoted and anti-Semitic.”

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But while Dershowitz noted that Ellison has claimed he was unaware of Farrakhan’s anti-Semitism, he said this was “not a credible statement” as Farrakhan “proclaimed it on every occasion.” Ellison, he said, “is either lying or he willfully blinded himself to what was obvious to everyone else. Neither of these qualities makes him suitable to be the next chairman of the DNC.”

Dershowitz also noted that Ellison has said Israel was founded under “dubious circumstances” and has been an outspoken critic of many Israeli governmental policies throughout his career in public life, most notably for its continued settlement enterprise and military response during periods of intensified conflict with the Palestinians.

“Ellison is now on an apology tour as he runs for DNC chairman, but his apologies and renunciations of his past association with anti-Semitism have been tactical and timed to his political aspirations,” Dershowitz added. “I do not trust him. I do not believe him.”

Ellison’s actions in Congress have troubled some in the pro-Israel community. Though Ellison has rejected the movement to boycott, sanction and divest from Israel and expressed support for the two-state-solution, he sought to accommodate a more sympathetic reading of the UN’s Goldstone Report on the 2009 Gaza war that Israel, Jewish groups and most of the Congress rejected as a one-sided attack on Israel. The congressman also voted against funding for Israel’s Iron Dome missile defense system in 2014.

In December Israeli-American businessman Haim Saban, a major Democratic Party funder, slammed the lawmaker as “an anti-Semite and anti-Israel individual” whose election as chairman of the Democratic National Committee would be a “disaster” for the party’s relationship with Jews. In January, however, Ellison said he had talked to Saban and that they were “on the road” to mending ties.

Ellison has also retained the support of the left-wing Middle East policy group J Street as well as some 300 Jewish leaders of liberal groups who signed a letter in support of him.

Dershowitz insisted that breaking hard-left following the recent election would get Democrats nowhere, noting that in the UK Labour leader Jeremy Corbyn “could not get elected dog catcher.”

If the party does indeed make the “self-destructive” decision to elect Ellison, he said, he would leave the party after 60 years, becoming an independent.