Insurgents determined to derail this weekend's historic referendum bombed an office of Iraq's largest Sunni Arab political party on Friday, police said, after the group dropped its opposition to the draft constitution. No one was wounded by the roadside bomb just outside the Iraqi Islamic Party office in Fadhal, a district of central Baghdad. But the rare attack against the group by Sunni-led insurgents appeared aimed at punishing it for deciding to end its "no" campaign against the referendum after lawmakers agreed Wednesday night to several amendments to the constitution designed to win Sunni support in Saturday's vote. On Thursday, Iraqi Islamic Party banners urging a "no" vote had been removed from where they hung near monuments such as the capital's main Grand Imam mosque. "This attack by insurgents against the Islamic Party was expected because of its new stand toward the referendum," Iraqi army Maj. Salman Abdul Yahid said in an interview. "Insurgents had threatened to attack the group and its leaders to get revenge." A senior party official, Alaa Makki, condemned the attack, saying it won't stop the group's efforts to "use the political process to fight terrorism and promote stability in Iraq." But other Sunni Arab parties still oppose the charter. They fear it would divide Iraq into three separate districts: powerful mini-states of Kurds in the north and majority Shi'ites in the south, both capitalizing on Iraq's oil wealth. By contrast, many Sunnis fear, their minority would be left isolated in central and western Iraq with a weak central government in Baghdad. On Wednesday night, the National Assembly endorsed last-minute changes to the draft constitution worked out by Shi'ite, Kurdish and Sunni powerbrokers that will allow a new parliament scheduled to be elected in December to adopt amendments to the constitution. But that compromise didn't please everyone. Industry Minister Osama al-Najafi, a Sunni member in Iraq's Cabinet, predicted on Friday that Iraqis will reject the document. "If the Iraqis are given the opportunity to vote freely, they will say no," he said during a visit to Malaysia. "The constitution does not represent the aspirations of all Iraqi people." The draft constitution being considered by voters is expected to pass on Saturday. It requires a majority and will be adopted unless two-thirds of voters in three provinces say "no." Sunnis only have a majority in four of Iraq's 18 provinces. Friday's roadside bomb attack against the Iraqi Islamic Party came as coalition forces closed Iraq's borders and its international airport in Baghdad in another effort to improve security to protect voters. On Thursday, a new 10 p.m. to 6 a.m. curfew was imposed, and a four-day holiday started across the country, closing government offices and schools. All civilian vehicles will be banned on Saturday as Iraqis are expected to walk by the thousands to 6,100 polling centers in Iraq. The referendum is an important step in the Bush administration's efforts to one day withdraw the tens of thousands of US forces from Iraq after establishing a stable and democratic government that is strong enough to fight the country's deadly insurgent groups. Coalition forces have warned of a spike in attacks by terrorists ahead of Saturday's vote, and nearly 450 people have been killed in violence over the past 19 days, often by insurgents using suicide car bombs, roadside bombs and drive-by shootings. Many other Iraqis have been kidnapped and killed, with their bodies abandoned in remote areas. Hundreds of Iraqi police and army troops have fanned out across Baghdad, setting up checkpoints and fortifying polling stations with barbed wire and blast barriers. An eerie calm has settled over Baghdad and other cities, with little traffic on the streets, few pedestrians and many shops closed. In Shi'ite areas of Baghdad, hundreds of posters and banners urging a "yes" vote were plastered on many walls and shop windows. Iraq's top Shi'ite cleric, Grand Ayatollah Ali al-Sistani, has ordered his followers to approve the constitution. But few such posters hung in mostly Sunni districts of the city. In the so-called Triangle of Death, a mainly Sunni area south of Baghdad that is known for kidnappings and killings, there was no sign of posters either. On Thursday, Iraqi troops searched cars under the watchful eyes of comrades manning machine-gun positions. US helicopters hovered over the area. Traffic on the road through the "triangle" was thin. "I will vote 'yes' so as to isolate the troublemakers," said Faisal Galab, a Sunni Arab sheik from the town of Youssifiyah, about 12 miles (19 kilometers) south of Baghdad. "I have asked my family and clan to vote 'yes.'" Also Friday, a US military convoy was targeted by another roadside bomb; injuries were reported. In Baghdad, US military spokesman Maj. Gen. Rick Lynch provided an upbeat assessment of the security situation ahead of the vote, arguing that the insurgent danger was far less than on the eve of the Jan. 30 parliamentary election. Also, Iraq's security forces total 200,000 now, compared to 138,000 in January, Lynch said. But he still expects a referendum spike in attacks. "The insurgents have declared war on democracy and they're going to conduct horrific acts of violence," he said.