Shas is urging Prime Minister Benjamin Netanyahu to halt a bill under which the state would recognize IDF-sponsored conversions to Judaism that would not need the Chief Rabbinate's approval.
The Ministerial Committee for Legislation approved the Israel Defense Forces conversion bill earlier this week.
Shas, however, wants to promote a bill mandating that conversions be handled by the Chief Rabbinate.
Netanyahu stopped the process on the latter bill in July because it could "create a rift in the Jewish people."
Shas' spiritual leader, Rabbi Ovadia Yosef, backs Sephardic Chief Rabbi Shlomo Amar's stance against the bill on IDF-sponsored conversions.
Amar recently told government ministers from the Shas party that he strongly opposes a bill that would deny the Chief Rabbinate discretion on whether to approve conversions in the army.
Shas said it would work to stop the bill, which it says goes against its coalition agreement with Likud.
"We demand a conversion law. Conversions can't just go along without a guiding hand. With all due respect, the authority has to be with the Chief Rabbinate of Israel," said Minister without portfolio Meshulam Nahari, a member of Shas.
"That's the halakhic [Jewish law] authority in the state, and it's natural that conversions, both military and civilian, should go through it," he told Haaretz.
Amar has said he would not approve conversions in the IDF as long as the conversion bill he favors has not been passed.
The conversion bill that Netanyahu stopped gives authority for conversions to municipal rabbis on the premise that this would simplify the process. But that bill would also give the Chief Rabbinate sole authority to approve which rabbis conduct conversions and to oversee the process.
The IDF conversion bill, meanwhile, aims to prevent municipal marriage clerks who heed stricter ultra-Orthodox rabbis from refusing to register the marriages of people converted in the IDF. Such rabbis have refused to register marriages, citing doubts about the extent the IDF program adheres to Jewish law.
The Ministerial Committee for Legislation approved the bill on condition that Amar sanction it, making its chances of passing a cabinet vote nil. Nevertheless, Shas is concerned that it might come up for a vote at the upcoming cabinet meeting.
Amar wrote Netanyahu that he opposed the law because it showed "contempt" for soldiers converted in the IDF and "a clear trend toward the ruin of religion in Israel, the likes of which no other government has done."
Amar also wrote Netanyahu that if the IDF conversion bill passed he "could no longer handle any matters of conversion and would not take responsibility for them."
Tzohar, a group of modern Orthodox rabbis, said it supported the conversion bill because the Chief Rabbinate had not done enough against rabbis and rabbinic judges who did not recognize conversions and had "caused converts to suffer."
Prof. Yedidia Stern, vice president of research of the Israel Democracy Institute, called the threat to stop conversions "a strategic opportunity to change the state's rules of conduct and place the entire conversion process in the hands of the state conversion system."
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