Baghdad, Iraq ? Under U.S. and U.N. pressure, Iraq’s Shiite-led parliament Wednesday reversed its last-minute electoral law changes, which would have ensured passage of a new constitution but which the United Nations called unfair.

Sunni Arab leaders who had threatened a boycott because of the changes said they were satisfied with the reversal and were now mobilizing to defeat the charter at the polls. But some warned they could still call a boycott to protest major U.S. offensives launched in western Iraq, the Sunni heartland.

Also, a bomb exploded at the entrance of a Shiite mosque south of Baghdad, killing at least 25 and wounding 87, as hundreds of worshippers gathered there for prayers at the start of the Islamic month of Ramadan and for the funeral of a man killed two days ago in a bomb blast at his restaurant.

It was the latest in a string of insurgent attacks – targeting Shiite Muslims in particular – aimed at wrecking the Oct. 15 referendum. Al-Qaida in Iraq, which has declared “all-out war” on Shiites, has called for stepped up violence during Ramadan. More than 270 people have been killed in the past 10 days.

Thousands of U.S. troops were waging two major offensives in western Iraq, the Sunni heartland, in an attempt to put down insurgents ahead of the vote.

The reversal of the election changes passed by parliament over the weekend was a political victory for U.N. and U.S. officials, boosting chances that Sunnis will see the referendum as fair and participate, thus giving the outcome credibility.

Yet that success restored the possibility that Sunnis will manage to veto the constitution, which would prolong Iraq’s political instability. The United States in particular is eager to see the passage of the charter, seen as key to beginning the withdrawal of some U.S. forces.

Saleh al-Mutlaq, a top Sunni politician, said the boycott threat over the election law was lifted. “With this result, the Sunni Arabs will be able to defeat the constitution,” he told The Associated Press. “I am sure if there is honesty (in the election process) 95 percent of the Sunni Arabs will vote ‘no.”‘

But he warned that Sunnis might still call a boycott if the U.S. offensives do not end soon, saying the turmoil will suppress Sunni voting.

“If the ongoing military operations continue, we will have no other choice but to call for a boycott simply because we will be having no role to play in the political process to tell our opinion about the constitution,” he said.

U.S. commanders have said operations Iron Fist and River Gate, in towns along the Euphrates River, will be finished ahead of the referendum. The twin offensives aim to break insurgents’ hold their so residents feel safe going to the polls.

Meanwhile, members of the ruling Shiite-led coalition lay the groundwork to challenge the results if Sunnis succeed, expressing fears insurgent violence could prevent pro-constitution voters from going to the polls, swinging key Sunni provinces toward rejection.

While parliament reversed voting rule changes that would have made it nearly impossible for Sunnis to defeat the constitution, it also underlined that it would appeal voting results if it believed insurgent attacks or threats had affected the balloting.

The Shiite-led parliament ton Sunday had issued a new interpretation of the rules, saying two-thirds of registered voters had to vote “no” – not two-thirds of those casting ballots. That raised the bar for rejection, and outraged Sunnis threatened a boycott.