Mrs Clinton is campaigning ahead of Pennsylvania's 22 April primary
Hillary Clinton has vowed to fight on in the contest to be the Democrats' presidential candidate, comparing herself to the film character Rocky.
Speaking in Philadelphia, where the film Rocky was set, she said: "When it comes to finishing a fight, Rocky and I have a lot in common. I never quit."
Some senior Democrats have called for Mrs Clinton to quit the race. Rival Barack Obama has dismissed that idea.
Meanwhile, likely Republican nominee John McCain is visiting Virginia.
The Arizona senator, who should be confirmed as his party's presidential candidate at the Republican National Convention in September, is on a tour of places that played an important role in his past.
Campaigning in Pennsylvania ahead of the state's 22 April primary, Mr Obama assailed Mr McCain on the economy, accusing the Republican of ignoring middle-class families to favour the wealthy.
'Never give up'
Senator Clinton's speech to the Pennsylvania AFL-CIO, a federation of labour unions reiterated her determination to stay in the race for the Democratic nomination.
"I never give up. And neither do the American people," she said.
DEMOCRATIC DELEGATE RACE
BARACK OBAMA: 1,631
HILLARY CLINTON: 1,501
Delegates needed to secure nomination: 2,024.
Source: Associated Press, as of 1 April
"But just as it's getting time to vote here in Pennsylvania, Senator Obama says he's getting tired of it. His supporters say they want it to end."
Her comments followed a call last week by senior Senator Patrick Leahy, a supporter of Mr Obama, for her to step aside.
Speaking to a Pittsburgh radio station on Tuesday, Mr Obama again stressed that he did not back that notion.
"I've said for the last three days that I think that Senator Clinton should stay in the race as long as she wants... she has every right to compete and I'm looking forward to competing against her."
Nancy Pelosi, Democratic speaker of the US House of Representatives, told ABC News it was important to get behind one candidate if the party expected to win the White House in November.
However, while saying she was keen to see a resolution soon, she made it clear she was not urging Mrs Clinton to concede.
The prospect of Mrs Clinton staying in the race right up until August's Democratic convention terrifies some senior Democrats, says the BBC's North America editor Justin Webb.
Former New York Governor Mario Cuomo has suggested the two candidates should agree that whoever comes second should be the vice-presidential choice.
This would assure both the Clinton and Obama camps of a prize at the end of the day, our correspondent adds.
The latest opinion polls suggest Mrs Clinton is leading Mr Obama by more than 10 points in Pennsylvania.
Barack Obama has criticised John McCain over his policies on Iraq
However, she is trailing the Illinois senator in the number of delegates needed to obtain the nomination at the national party convention in August, with 1,501 delegates to his 1,631.
To secure the nomination, the winner must secure 2,024 delegates - which correspondents say neither candidate will be able to do by winning delegates in the remaining primary elections alone.
If Mrs Clinton manages to win a larger share of the votes cast in the Democratic contests overall, it is thought that she could secure the backing of the so-called super-delegates who could tip the balance.
Analysts say a bitter, drawn-out fight between the two contenders could damage the eventual nominee's chances of beating Mr McCain in November.
Both Democrats have turned their attention from sparring with each other to attacking Mr McCain in recent days.
HAVE YOUR SAY
The Democrats have not yet figured out that it is better to concede in style for the mutual benefit of their party
Mrs Clinton accused Mr McCain of letting down struggling US families on Tuesday, saying he offering "more of the same" economic policies as the Bush administration.
Mr Obama, on a six-day bus tour of Pennsylvania, said voters should be more interested in where Mr McCain stood on Iraq and the economy than in his personal history.
Mr McCain, who visited his old high school in Virginia on Tuesday, will visit the Maryland naval academy where he trained as a cadet on Wednesday.
The visits are intended to remind US voters of his years of military and public service, at a time when news coverage is being dominated by the Democratic battle.
The 71-year-old senator also showed his lighter side with an April Fool's Day appearance on CBS's "The Late Show with David Letterman".
Responding to jibes about his age, told Mr Letterman he looked "like a guy whose laptop would be seized by the authorities".