Mr Caplin, a former defence minister in Tony Blair’s government, took over the role last June in the wake of Jeremy Newmark’s resignation.
With the antisemitism crisis in Labour under Jeremy Corbyn, the past year has proved to be one of the most turbulent in JLM’s 116-year history.
Mr Caplin took centre stage at last week’s Emergency General Meeting in London to discuss whether JLM should remain affiliated with the Labour Party ahead of an official vote on the decision at the group’s AGM on April 7.
Several senior figures connected to JLM told the JC said they feared the meetings - where members voted to stick with Mr Corbyn’s party - sent out the wrong message and hid a growing element who believe disaffiliation is inevitable.
Mr Caplin was criticised the following day when he appeared to reject claims that the Labour leader could be labelled antisemitic during an appearance on Nick Ferrari’s LBC radio show.
The interview coincided with the Equalities and Human Rights Commission announcing that it was investigating Labour over its handling of complaints about antisemitism after JLM submitted a 1,000 page dossier of its members' accounts of alleged mistreatment.
As JLM chair, Mr Caplin has managed to forge a close working relationship with Board of Deputies President Marie van der Zyl.
Last September’s annual JLM conference at JW3 made headlines when former Prime Minister Gordon Brown called on Mr Corbyn to adopt the IHRA antisemitism definition in full.
But the JC had revealed how Mr Caplin only weeks earlier had infuriated senior JLM figures by meeting Labour General Secretary Jennie Formby just a day before the party announced a hugely controversial attempt to amend the IHRA definition, which triggered a huge row and eventual climbdown.
Mr Caplin furiously denied suggestions he had “waved through” Labour’s new guidelines on a “red line communal issue”.
It is understood the parliamentary chair Luciana Berger, and national secretary Peter Mason were among those to raise concerns about his meeting with Ms Formby.
There was a second clash when it emerged that Mr Caplin had attended a second meeting with Ms Formby on his own. JLM policy was to attend official meetings with at least one other person from the group to ensure a proper record of the talks can be logged.
In an earlier incident Mr Caplin was forced to apologise “unreservedly” to Holocaust expert Prof Deborah Lipstadt after he launched an astonishing attack on her comments on Labour’s antisemitism crisis.
The JLM chair said there were "too many people who are outside the Labour Party like an American academic who come over and say this", during an interview on LBC last July.
His remarks came after Prof Lipstadt used a Holocaust Educational Trust conference speech to blame Jeremy Corbyn’s Labour for a rise in what she said was “softcore” Holocaust denial.
The JC has learned that Mr Caplin's comments caused such anger among JLM members, that one member of the group’s ruling national executive committee threatened to resign in protest.
Mr Caplin told the JC he was planning to make a full statement at the end of the week regarding the challenge. JLM activist Colin Appleby has also announced he will stand.
Voting to decide the JLM chair role will take place at the AGM on April 7.
It is understood the future of Luciana Berger as parliamentary chair will also be discussed as she quit Labour for The Independent Group last month in protest at “institutional antisemitism” under Mr Corbyn.
The announcement by TIG spokesman Chuka Umunna that they have contacted the electoral commission over registering the group as a political party would make Ms Berger illegible to continue in the role.
Labour MP Ruth Smeeth is hotly tipped to replace her.
