Barack Obama has been confronted for the second time in 48 hours with efforts by conservative Republicans to convince voters that he is a Muslim and anti-Semite.

In a taste of what the Democratic presidential frontrunner can expect from Republican surrogates if he wins his party's nomination, he has been frequently referred to this week by his full name - "Barack Hussein Obama" - and accused of consulting with anti-Semitic advisers.

John McCain, the presumptive Republican nominee, has twice been forced to rebuke members of his own party after the attacks. They followed the appearance of a photograph of Mr Obama dressed in traditional Somali dress, including a turban, on the Drudge Report website on Monday. The picture was taken during a visit to Africa by Mr Obama in 2006.

Matt Drudge, the website's owner, claimed the photograph was leaked by the Hillary Clinton campaign. The former First Lady said in a debate on Tuesday night that as far as she knew it did not come from her campaign.

In that debate, Mr Obama was asked whether he would renounce support from Louis Farrakhan, the Nation of Islam leader who has praised Mr Obama. Viewers were reminded that Mr Farrakhan had once called Judaism a "gutter religion". Mr Obama denounced - and eventually rejected - Mr Farrakhan's support, but the issue is bound to return to the campaign if he wins the nomination.

Mr Obama is a Christian who has never worshipped at a mosque. He was raised by a secular mother and has been a member of the United Church of Christ in Chicago for 20 years. His Kenyan father, who left the family when he was two, was Muslim.

Yet the rumours of his Islamism have been circulating for over a year, beginning with an anonymous and untraceable email. The internet gossip reached such levels that Mr Obama began debunking it in his stump speeches. His website has a section entitled, "Barack is not and never has been a Muslim". He told Jewish leaders in Ohio this week: "If anyone is still puzzled about the facts, in fact I have never been a Muslim."

Mr McCain's aides are concerned that such attacks by overzealous activists will backfire. At a McCain rally in Ohio this week Bill Cunningham, a conservative talk radio show host, referred three times to "Barack Hussein Obama". Mr McCain, who had not yet arrived at the event, later denounced the remarks.

On Wednesday, the Tennessee Republican party website published the photo of Mr Obama in Somali dress, called him "Barack Hussein Obama", and accused him of being an anti-Semite.

Mr McCain denounced the website. He said: "If I am the nominee of the party, I will obviously assure everyone within my party knows that this has got to be a respectful debate."

The controversy flared as another black Democratic superdelegate - Texas's Senfronia Thompson - defected from Hillary Clinton to Mr Obama. She followed the switch on Wednesday by Georgia's John Lewis, one of America's most prominent African American congressman.

A new poll also showed Mr Obama opening a four-point lead in Texas ahead of the critical contests next Tuesday there and in Ohio. Aides to Mrs Clinton believe it will be difficult for her to stay in the race unless she wins both.

Meanwhile, Michael Bloomberg, the New York mayor and billionaire, announced he would not launch a presidential bid as an independent, after showing strong signs of jumping into the race.

President Bush interjected himself into the race to succeed him. Mr McCain has lambasted Mr Obama over remarks he made in Tuesday's debate when he said he would reserve the right to send troops back into Iraq if "al-Qaeda is forming a base in Iraq". Mr McCain retorted: "I have some news. Al-Qaeda is in Iraq. It's called al-Qaeda in Iraq."

Mr Bush called Mr Obama's comments "interesting". He also criticised Mr Obama for saying he would meet with leaders from rival nations.

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