Clinton wins key Democratic vote
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Hillary Clinton: 'You know you can count on me'
Hillary Clinton has beaten rival Barack Obama in a critical vote in the state of Pennsylvania as the two battle to be the Democrats' presidential candidate.
Speaking after her convincing 10-point win, she said Americans deserved "a president who doesn't quit".
Although this victory keeps her campaign alive, Mrs Clinton's win is not enough to turn the tables on her rival, correspondents say.
Mr Obama said he had done well to gain ground on Mrs Clinton in Pennsylvania.
With 99% of returns counted, Mrs Clinton was leading Mr Obama by 55% to 45%.
No quitting
The former first lady told cheering supporters at a victory rally in Philadelphia that she had beaten a "formidable opponent" who had outspent her three-to-one.
"Some people counted me out and said to drop out, but the American people don't quit and they deserve a president who doesn't quit either," she said.
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Barack Obama: 'New voters will lead Democrats to victory'
"Because of you, the tide is turning."
She also appealed for more donations to her campaign war chest, saying it was the only way she could continue to compete with a rival who was outspending her "massively".
Mr Obama congratulated Mrs Clinton as he addressed a rally in Indiana, one of the next states to hold a primary vote, but also said his campaign had done better than expected in Pennsylvania.
"There were a lot of folks who didn't think we could make this a close race when it started," he said.
"Six weeks later, we closed the gap. We rallied people of every age and race and background to our cause."
The BBC's Jonathan Beale in Washington says Mrs Clinton has gained a clear victory that keeps her presidential hopes alive.
DEMOCRATIC DELEGATES
Barack Obama: Pledged delegates: 1,415
Super-delegates: 233
Total: 1,648
Hillary Clinton: Pledged delegates: 1,251
Super-delegates: 258
Total: 1,509
Source: AP estimates ahead of 22 April primary
While her win is not enough to turn the tables on her rival, who still retains the overall lead and has much more cash to spend, it suggests her perseverance and resilience is paying off, our correspondent adds.
Exit polls for US media suggest Mrs Clinton won by taking the votes of blue-collar workers, women and white men, the Associated Press reported.
Women - who have tended to favour Mrs Clinton - made up 60% of Democratic voters in the state, according to the exit polls.
Mrs Clinton also won the support of six out of 10 of those who decided which of the pair to back within the last week - a period during which campaign rhetoric sharpened.
The economy once again emerged as the most important issue for voters, with more than 80% of those surveyed saying the nation was already in a recession.
'Big state'
With four million registered Democrats, and 158 pledged delegates to the Democratic Party's nominating convention in August, Pennsylvania was the last of the big states to hold a primary.
Although Mrs Clinton is behind in the delegate count and in the total votes cast, she has won most of the big state contests.
With the delegates split in proportion to the vote, neither candidate is expected to win sufficient pledged delegates to seal the nomination in the remaining primaries, and the two are courting 800 or so unelected "super-delegates".
Mrs Clinton's argument - which she hopes will sway the super-delegates - is that only she will be able to secure wins in critical large states come November's presidential election.
Mr Obama has dismissed the big-state argument, saying there was "no chance" of the Democrats losing New York or California in the presidential election, no matter who the candidate was.
The Republican Party also held a primary in Pennsylvania on Tuesday, with presumptive nominee John McCain set to take all the delegates on offer.
Mr McCain spent the day in Ohio - expected to be a key battleground state in November - where he spoke of the need to create new opportunities to replace lost blue-collar jobs.
Iran warning
On Tuesday, Mrs Clinton emphasised what she says is a strength - her leadership ability and foreign affairs credentials.
HAVE YOUR SAY Let them fight it out, I do not care who wins; either candidate will be a strong contender Wayne, Idaho
As the contenders appeared on the US morning talk show circuit, Mrs Clinton was asked how she would respond if Iran launched a nuclear attack on Israel, and replied with a stark warning.
"If I'm the president, we will attack Iran... we would be able to totally obliterate them," she told TV network ABC.
"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."
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.
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