Democratic presidential contender Hillary Clinton is sounding a defiant tone in response to calls from some supporters of rival Barack Obama that she quit the presidential race. Meanwhile, the presumptive Republican candidate, Senator John McCain, has begun a week-long tour aimed at reintroducing himself to American voters. VOA National Correspondent Jim Malone reports from Washington.

The two Democratic candidates campaigned in Pennsylvania in advance of the primary there on April 22.

Opinion polls give Clinton a lead in Pennsylvania and the New York Senator has been quick to cite them as she fends off suggestions from Obama supporters that it is time for her to quit the race.

"You know, there are some folks saying we ought to stop these elections," she said. "I did not think we believed that in America."

Senator Clinton continues to get help from her husband, former President Bill Clinton. He argues against the view that the lengthy nomination battle will hurt the Democratic Party.

"We are strengthening the Democratic Party. Chill out! We are going to win this election if we just chill out and let everybody have their say," she said.

Some prominent Obama supporters have urged Clinton to quit because they believe there is no way she can catch the Illinois Senator in the race for party nominating delegates.

But Senator Obama has yet to join those calling on Clinton to withdraw.

"My attitude is that Senator Clinton can run as long as she wants," he said. "Her name is on the ballot and she is a fierce and formidable competitor."

Some Democrats fear the party could be torn apart if the nomination race goes all the way to the party's national nominating convention in Denver in late August.

Analysts have suggested party leaders may eventually turn to a respected figure like former Vice President Al Gore to mediate between Obama and Clinton before the convention. But Gore showed little interest in that in an interview with the CBS program 60 Minutes.

"I am not applying for the job of broker," he said.

While the Democrats focus on Pennsylvania and nine other upcoming nominating contests that go into June, Republican John McCain has begun an effort to reintroduce himself to the American public.

McCain began a biographical campaign tour in Mississippi, where he recalled the influence of his father and grandfather, both of whom were admirals in the U.S. Navy.

"My grandfather was an aviator and my father was a submariner. They were my first heroes," he said. "I have been an imperfect servant of my country for many years. But I am their son, and they showed me how to love my country, and that has made all the difference for me, my friends, all the difference in the world."

Later this week, McCain is scheduled to speak in Maryland at the U.S. Naval Academy, from which he graduated, and a Naval air station in Florida where he trained as an aviator.

McCain spent five-and-one-half years as a prisoner of war after being shot down over North Vietnam in 1967.