Among the many supposedly iron rules of US politics which Donald Trump has twisted, bent and snapped is the one which states that when you are in a hole, you should stop digging.

Having initially failed to disavow support from David Duke, the Republican presidential frontrunner last week attempted to explain himself by drawing a bizarre comparison between the Ku Klux Klan and the Federation of Jewish Philanthropies.

If he becomes his party's standard-bearer in November's elections, Mr Trump's comments will become one more exhibit in any post-mortem into why the Republicans failed to make any inroads into the solidly Democratic Jewish vote.

Despite their perennial optimism that they can detach Jews from their Democrat moorings, the Republicans' problems go much deeper than having a xenophobic billionaire as their presidential candidate. Pew Research Centre polling in 2014 - when Mr Trump did not even figure in polls of likely 2016 candidates - showed the Democrats nearly 40 points ahead of the Republicans.

The Republicans' wishful thinking rests, in large measure, on the notion that they simply need to paint themselves as better friends of Israel in order to win significant Jewish support. But, as Haaretz's US editor, Chemi Shalev, argued: "One of the prime factors that has driven the Republicans to unequivocally embrace Israel … is Christian Evangelicals."

Bernie shows jewish pride

Jewish presidential candidate Bernie Sanders last Sunday hit back at a TV host who asked him if he was intentionally downplaying his Judaism. Mr Sanders told the CNN news anchor Anderson Cooper: Being Jewish is so much of what I am. Look, my father's family was wiped out by Hitler in the Holocaust. I know about what radical and extremist politics mean.

The Republicans' association with Evangelicals is, however, a major turn-off for Jews. As Pew also showed, while Evangelical Christians feel more positive towards Jews than they do towards any other religious group, this warmth is unrequited. Moreover, the "litmus tests" that the "religious right" impose on Republican candidates - such as opposition to gay marriage and abortion - alienate Jewish voters, who overwhelmingly support both.

Few candidates in 2016 epitomise the link between Evangelicals and the Republicans more than Texas Senator Ted Cruz, who has been stealthily amassing convention delegates. Over the past week, his campaign has been boosted by victories in the Idaho primary and caucuses in Kansas and Maine. His momentum has been slowed, however, by Trump's wins in Louisiana, Michigan and Mississippi.

The Tea Party favourite is one of Israel's most avid defenders in the Senate; his interest was sparked by the Entebbe raid which he later said "struck me as a profoundly Texan approach to an act of terrorism". While his efforts to woo conservative Jews have met with some success, many liberal and secular Jews regard his hard-right policies with distaste and view with suspicion his claim to share "a great many values with the Jewish community". Certainly, Mr Cruz's defence of a pastor who claimed that God had sent Hitler to hunt Jews dented those credentials.

Next Tuesday, Florida - the state with the third highest proportion of Jews - holds its primaries. Mr Trump's main challenger there is Senator Marco Rubio. Despite hammering him for his alleged lack of support for Israel, Mr Rubio trails the billionaire there by double-digits. Defeat in his home state is almost certainly likely to drive the Florida Senator from the race. Mr Rubio's problem is that few Jews will participate in the Republican vote: in 2012, only one per cent of the state's GOP primary voters were Jews. By contrast, they made up nine per cent of voters in the last Democrat presidential primary in the state eight years ago, when Hillary Clinton won the support of nearly 60 per cent of them.

There is one glimmer on the horizon for the Republicans. More socially conservative, Orthodox Jews, whose number is growing faster than other groups, are more open to Republican wooing. In 2013, 57 per cent said they backed the GOP. But, come November, even many of these may baulk at a thrice-married casino-owner who claimed to be "neutral" on the Israeli-Palestinian conflict.