Clinton's poll day threat to Iran
Hillary Clinton has issued a stark warning to Iran, as Democrats in Pennsylvania vote to choose between her and Barack Obama to run for president.
She said the US would attack, and could "obliterate" Iran, if it launched a nuclear strike on Israel.
Mrs Clinton has been playing up foreign affairs and leadership as she tries to make up ground in the Democratic race.
She leads polls in Pennsylvania, the largest remaining state, but analysts say her hopes depend on a big victory.
A Zogby survey released on Tuesday showed Mrs Clinton leading Mr Obama by 10%, while an InsiderAdvantage poll had her 7% ahead.
Polling stations opened at 0700 (1100 GMT), with results expected soon after they close at 2000 (2400 GMT).
Hillary Clinton's threat to Iran
As the candidates appeared on the US talk show circuit on Tuesday morning, a row erupted when Mrs Clinton was asked how she would respond if Iran launched a nuclear attack on Israel.
She replied that: "If I'm the president, we will attack Iran... we would be able to totally obliterate them.
"That's a terrible thing to say, but those people who run Iran need to understand that, because that perhaps will deter them from doing something that would be reckless, foolish and tragic," she told TV channel ABC.
In response, Mr Obama said: "Using words like 'obliterate' - it doesn't actually produce good results, and so I'm not interested in sabre-rattling."
He said only that Iran should know he would respond "forcefully" to an attack on any US ally.
The US fears Iran is trying to acquire nuclear weapons, and could use them against Israel. Iran insists its nuclear programme is solely for power generation. Mrs Clinton's tough talking over Iran is part of her strategy, to emphasise her foreign policy experience as she fights for every last vote available in Pennsylvania, says the BBC's Jack Izzard.