Today the Likud party is voting in an internal primary to determine who will head it in the years to come. The results are a foregone conclusion: Benjamin Netanyahu will win by a wide margin. And yet, as he and his associates have been warning Likud members in recent days in an effort to get out the vote, it may not be wide enough for comfort.

This is not only because the single candidate running against Mr. Netanyahu, Moshe Feiglin, has no real roots in Likud and joined it only a few years ago with the open intention of staging a hostile takeover. It is also not only because Mr. Feiglin’s views on the Israeli-Palestinian conflict make Mr. Netanyahu look like an extreme dove. It is in addition, and most of all, because these views are rooted in Jewish religious belief and in a religious perspective, according to which the entire biblical land of Israel belongs to the Jewish people by a divine decree that must be implemented by any and all means. Mr. Feiglin, that is, who first came to national prominence as a leader of street demonstrations against the Oslo agreement in the months before the Rabin assassination, has an ideology no different from that of the most militant members of the religious settlement movement. This is an ideology that one can consider, as I do, potentially catastrophic for the future of Israel without denying that it has a right to be represented in Israeli political life. And indeed it is represented, with nine seats in the current 120-member Knesset, held by an electoral bloc formed by an alliance of the National Religious Party, whose voters are almost entirely Orthodox Jews, and the National Union Party, whose voters are heavily so.

It is in this alliance that Moshe Feiglin naturally belongs. But Mr. Feiglin’s ambitions extend well beyond being a shareholder in a small Knesset bloc that can at the most exert pressure on the bigger parties that ultimately decide Israel’s fate. He would like to own a controlling share of one of those bigger parties — or at least, a sizable chunk of it.

To this end he has adopted a simple but strategy, appealing to right-wing religious Jews like himself to join the Likud and vote in its primaries. Thousands of them have done this, both as individuals and in organized groups, even though some may also be registered in other parties and many may not vote for Likud at all in national elections. And since primaries usually have low turnouts that give an advantage to the ideologically highly motivated, Mr. Feiglin has done well, winning 15% of the vote in 2005 and — so the latest polls show — threatening to win as much as 30% this time.

The effect on Likud of such an outcome could be horrendous. It could give the Feiglin faction an entrenched position in Likud’s institutions; encourage many more Israelis who think like him to join the party; assure him and his followers high places on the its Knesset list; dissuade secular Israelis from voting for it; and thus torpedo Benjamin Netanyahu’s chances of being elected prime minister in the next national elections. It’s no wonder that Mr. Netanyahu, though he awoke to the danger late in the day, has been running scared.

But it’s not just the career of Benjamin Netanyahu, who has been shown by the polls to be the current prime-ministerial favorite of a plurality of Israelis that is threatened. It’s the future of his party as well — and beyond that, the future of the secular Israeli Center and Right. For years now they have been losing strength, pinched by the religious Right on one side and the secular Left on the other, and a strong showing in today’s primaries by Mr. Feiglin could help sink Likud as the Center-Right’s flagship.

This would be highly unfortunate, because if Israel is not to turn into a society hopelessly split between the secular Left and the religious Right, it needs a secular Center-Right more than ever. Only a political body can mediate between the extremes, and only it can provide rational alternatives to the policies of the Left, those of the religious Right being by definition a-rational, and often irrational. Faith in the God of Israel may be a praiseworthy trait, but it is hardly a dependable guide to making fateful policy choices.

It is worth remembering, at a time when the Right in Israel — as in America, but even more so — has become increasingly associated with religion, that its founding father and greatest historical figure, Vladimir Jabotinsky (1880-1940), whose Zionist Revisionist Party established in the 1920s was the grandfather of today’s Likud, didn’t have a religious bone in his body. Although Jabotinsky was always respectful of Jewish religious tradition and values in a way that the Zionist Left of his age rarely was, he never shared them and was never personally motivated by them, and his Jewish nationalism went hand in hand with a deep commitment to European liberal democracy. Whatever he might think of the Israeli-Palestinian conflict were he alive today, and whatever solutions for it he might support, one can rest assured that he would leave God out of it.

This is as it should be. There is a need for a strong political party in Israel today that is for a free market economy (as Jabotinsky was) that recognizes the importance of territory (as Jabotinsky did) that understands (as Jabotinsky was always aware) that this is a cruel world in which a Jewish state must never let down its guard, and that nevertheless wants this state (as Jabotinsky wanted it) to be free from religious bigotry and fanaticism and aligned with the traditions of European humanism. Bigotry and religious fanaticism are Moshe Feiglin’s natural elements. Let’s hope that Benjamin Netanyahu gets out the vote today.

Mr. Halkin is a contributing editor of The New York Sun.