Barack Obama's bid to become the first black American president could have taken a blow with the revelation that his aunt was an illegal immigrant in the US.

News about 56 year old Zeituni Onyango just three days before election is a chance for Republican presidential candidate John McCain to generate some last-minute negative headlines about Obama.

But Obama has denied having any knowledge about his aunt living in the US illegally, and said laws covering the case should be adhered to. A statement given to the Associated Press (AP) by Obama's campaign said: "Senator Obama has no knowledge of her status but obviously believes that any and all appropriate laws be followed."

Traveling with Obama in Nevada, Democrat campaign strategist David Axelrod declined to elaborate on the statement, but said: "I think people are suspicious about stories that surface in the last 72 hours of a national campaign."

AP found that Onyango, who is mentioned in Obama's personal memoir, had been instructed to leave the country four years ago by an immigration judge who rejected her request for asylum from her native Kenya. But the half-sister of Obama's late father is living in public housing in Boston.

She has even donated to Obama's campaign, with five small gifts totalling $260. Those donations may turn out to be illegal because she is not a US citizen.

Illegal immigration is a political minefield in US politics, and the story could be a vote-losing headache for Obama. Her refusal to leave the country does not make her unusual in the US - there are an estimated 10 million "illegals" - but the issue is hot especially because Obama and McCain both are trying to woo white working-class voters. Illegal immigrants are often accused of taking jobs from legal residents.

On Saturday, McCain was nudging into a slight lead in one element of a three-day tracking poll by respected pollster John Zogby. The overall survey, though, still gave victory to Obama by five points.

Obama will hold a rally in Chicago after the polls close on Tuesday night, and up to 1 million people could flood the city's streets. All police leave has been cancelled and off-duty firefighters have been told to keep their kit ready at home. The unspoken concern among some officials is that a surprise Obama loss - especially with most polls predicting a win for him - could lead to civil unrest.

Detroit and Los Angeles were also deploying extra police out of concern for heightened emotions raised by this election.

The Zogby poll found McCain's support was increasing among independents and working-class voters. It could prove that McCain's recent tactic of pushing blue-collar issues, symbolized by his adoption of "Joe the Plumber" as a campaign totem, is working. But virtually every other national poll suggests Obama has a healthy lead.

Questions:

1. What was the name of Barack Obama’s aunt?

2. Why is the issue hot?

3. Where and when will Obama hold a rally?

Answers:

1. Zeituni Onyango.

2. Because Obama and McCain both are trying to woo white working-class voters. Illegal immigrants are often accused of taking jobs from legal residents.

3. In Chicago after the polls close on Tuesday nightntries.