US Democratic presidential candidate Barack Obama is to leave the campaign trail for two days to visit his sick grandmother in Hawaii.

Madelyn Dunham, 85, who helped to raise Mr Obama, is said to be seriously ill.

Mr Obama has spoken of their strong bond and has referred to her in some of his most powerful campaign speeches.

BBC North America editor Justin Webb says despite Mr Obama's substantial lead over his rival, John McCain, his absence will make his staff nervous.

The 47-year-old Illinois senator has the advantage in the polls with two weeks to go until election day on 4 November, but is not sufficiently far ahead to be confident of victory,

 Senator Obama's grandmother, Madelyn Dunham, has always been one of the most important people in his life

At rallies on Monday, his Republican opponent pledged to do more to help homeowners defaulting on their mortgages, and said that, unlike Mr Obama, he would cut taxes to help create jobs.

Campaigning in the swing state of Missouri, Mr McCain accused his Democratic rival of misleading the electorate by plotting to put up taxes.

"After months of campaign trail eloquence... we finally learned what Senator Obama's economic goal is... he wants to 'spread the wealth' around," the 72-year-old Arizona senator said.

Mr Obama had been due to campaign in the battleground states of Wisconsin and Ohio on Thursday and Friday - both states analysts say he appears likely to win.

Instead he will attend a campaign event in Indianapolis on Thursday before taking a flight to Honolulu.

Obama campaign spokeswoman Jen Psaki said that on Friday, Mr Obama's wife, Michelle, would campaign for her husband in Ohio.

The Democratic nominee is expected to resume campaigning on Saturday.

Mrs Dunham was released from hospital late last week and her health had deteriorated "to the point where her situation is very serious", said Obama aide Robert Gibbs.  McCain insists it's not too late - he's invigorated at the moment, and seems to have renewed energy

"Senator Obama's grandmother, Madelyn Dunham, has always been one of the most important people in his life," Mr Gibbs said in a statement.

"Along with his mother and his grandfather, she raised him in Hawaii from the time he was born until the moment he left for college."

In a campaign advertisement this year, Mr Obama described Mrs Dunham as the daughter of a Midwest oil company clerk who "taught me values straight from the Kansas heartland".

She is also the "white grandmother" he referred to in a speech on race at the time questions were being asked about his relationship with his former pastor Rev Jeremiah Wright.

At the time, Mr Obama said he could no more disown Mr Wright than he could disown his white grandmother - who once confessed her fear of black men who passed her on the street.

The development came as Mr Obama made campaign stops in Florida on Monday, including a rare joint appearance with his former rival Hillary Clinton.

He accused the Republican camp of "ugly" campaigning, keeping his focus on the economy and those in the state who have been hit hard by the mortgage crisis.

New York Senator Clinton joined her fellow Democrat on stage and urged a rally of 50,000 supporters to "close the deal for Barack Obama".

It was the first time the pair - who were bitter rivals in the Democratic primaries - had appeared together since July.

Mr Obama has been pushing early voting on a major scale, using speeches, e-mails and advertisements placed in popular video games.

Thousands of Mr Obama's volunteers have been mobilised, including "town criers" riding on public buses, offering lifts to college students, and neighbourhood teams to give voters a push.

Meanwhile Mr McCain's campaign has opted to focus its efforts more selectively on supporters who do not always vote in presidential elections.

About a third of US voters are expected to cast their votes early this year.