With US mediation, Iraqi Shiâ€™ite and Kurdish officials negotiated with Sunni Arab leaders over last minute additions to the constitution, trying to win Sunni support ahead of next weekend's crucial referendum. But the sides remained far apart Sunday over basic issues - including the federalism that Shiâ€™ites and Kurds insist on - and copies of the constitution are already being passed out to the public. Sunni-led insurgents are trying to prevent Iraqis from voting with a wave of attacks over the past two weeks. The government has launched a campaign to convince Iraqis to go to the polls despite the threats - and despite calls by some Sunni Arabs for a boycott. "We think [a boycott] would weaken Iraq because the only way that Iraq can recover is done by concentrating on the political process, writing the constitution and participating in it," government spokesman Laith Kubba said. "Any act that calls for violence or boycotting would deviate the country from its course." Iraq's Sunni Arab leaders are calling on their followers to turn out in force to vote in the referendum - but to vote "no" to defeat a draft constitution they say will break Iraqi into pieces, with Shiâ€™ite and Kurdish mini-states in the north and south, and with the Sunni minority left poor and weak in a central zone. Though a minority, Sunnis can defeat the charter if they garner a two-thirds "no" vote in any three of Iraq's 18 provinces - and they have the potential to make that threshold in four provinces. But turnout is key, since they must outweigh Shiâ€™ite and Kurdish populations in some of those areas. Even with copies of the official text of the constitution being distributed to voters to consider before the polls, all sides were debating last-minute changes in a bid to swing some Sunnis to a "yes" vote. Kurdish leader Massoud Barzani met with Sunni Arab leaders Saturday and Sunday trying to convince them on the changes, officials from all sides said. The United States is eager to see the passage of the constitution, since its rejection would prolong Iraq's political instability for months - and could hamper the US military's plans to start pulling out some troops next year. But there appeared to be too wide a gulf to convince Sunni leaders to drop their opposition. While Shiâ€™ite and Kurdish parties were willing to make some cosmetic additions to the draft, they rejected what they called central changes sought by Sunnis, particularly ones aimed at reducing the strong powers the charter gives to regional administrations over the central government. "In general, there is no problem with making additions [to the draft] because it doesn't contradict the principles of the constitution. But the amendments the Sunna are demanding ... are basic changes in these issues that absolutely won't be accepted," said Shiek Jalaleddin al-Saghir, an official from the Shiâ€™ite -led United Iraqi Alliance, which dominated the government. The Sunnis seek in particular changes to the constitution's articles outlining the purging of members of Saddam Hussein's former Baath Party - most of whose major figures were Sunnis - and others allowing provinces to join together into "regions" under a single administration that would have considerable powers. "We don't want a federal system. It shouldn't be a system of regions, it's a system of provinces," Saleh al-Mutlaq, a prominent Sunni politician, said. He said the Sunnis want the articles on de-Baathification rewritten to "not single out the Baath Party."