Ever since he cast his very first vote, William Kristol has checked the box next to the name of the Republican party's presidential candidate.

That's hardly a surprise. The son of Irving Kristol, the so-called "godfather of neoconservatism", and the historian Gertrude Himmelfarb, Mr Kristol was reared in a Jewish home with a decidedly right-wing bent. Over the past three decades, he has made a name for himself as one of the party's leading intellectuals and operatives.

But, as he declared last week in the Weekly Standard, the conservative magazine he founded in the mid-1990s, Mr Kristol's GOP "presidential voting streak" is about to come to an end. Beyond the presumptive Republican nominee's "mixed bag of motley policies", he wrote, "it is clear that Donald Trump does not have the character to be president of the US".

Mr Kristol has now embarked on a last-ditch effort to recruit an independent candidate to oppose Mr Trump and Hillary Clinton in November, thus providing his fellow conservatives with an alternative to abstention or breaking ranks and voting for the Democrat candidate.

Having failed to persuade James Mattis, a highly respected former Marine Corps general, Mr Kristol now appears to have set his sights on convincing Mitt Romney, the unsuccessful Republican standard-bearer of four years ago, to throw his hat into the ring. The two met privately last week, with Mr Romney agreeing to aid the search for an independent candidate, although, for now, apparently sticking to his insistence that he will not undertake the task himself.

Among Jewish conservatives, Mr Kristol's refusal to support Mr Trump is by no means unique. Just as the Democrats' lurch to the left in the early 1970s sparked the rise of neoconservatism and led a group of high-profile Jewish intellectuals to flee to the Republican party, so the impending Trump coronation is splintering the uneasy coalition which underpinned the modern GOP.

Faced with a Republican candidate who is disinterring the party's isolationist and nativist past, many leading Jewish voices on the right are finding themselves politically homeless.

While the Republican Jewish Coalition endorsed their party's candidate last week - causing a handful of erstwhile Trump critics, such as former George W Bush press secretary Ari Fleischer, to fall into line - many more are refusing to do so.

Some, such as former Senator Norm Coleman, historian Daniel Pipes and Washington Post columnist Jennifer Rubin, have declared they will support neither Mr Trump nor Mrs Clinton. Others, such as Max Boot, a former foreign policy adviser to John McCain, look to be leaning towards backing Mrs Clinton.

Some, indeed, have already made the leap. Declaring that he would put aside his doubts and vote for the Democrat presidential candidate, the prominent neoconservative writer Joshua Muravchik wrote simply: "Trump has degraded American politics in a way unlike anything I have ever witnessed. I can't say enough bad things about him. His ignorance is staggering and his personality is revolting."