'Not ready for it to be over'
Clinton has given no sign she plans to exit the race before the final voting contests on June 3. Montana and South Dakota vote on that date, and Puerto Rico votes on June 1.
"There's some people who've been saying for months that this is over and every time they say it the voters come back and say, 'Oh no, it's not. We're not ready for it to be over,"' Clinton said while visiting a distillery in Loretto, Kentucky.
"You don't quit on people and you don't quit until you finish what you started," she said.
Clinton argues her strength in big states like Ohio and Pennsylvania that may be critical in a presidential election makes her the best candidate against McCain.
But for two straight days this week, she found herself sidelined as Obama sparred with McCain and Bush over Obama's vow to meet with leaders of US foes like Iran if elected.
Bush triggered Democratic outrage on Thursday by saying in a speech in Israel that some politicians would pursue the "false comfort of appeasement" by talking with militant groups such as Hamas and Hezbollah and with Iran.
Bush did not mention Obama but the Illinois senator accused the president and McCain of fear-mongering. McCain, an Arizona senator, joined Bush in criticizing Obama's willingness to hold direct talks with Iran.
Obama has said he would meet with Iranian President Mahmoud Ahmadinejad, but that he would not talk with Hamas because the Palestinian group is not a state and does not recognize Israel.
At a rally in Roseburg, Oregon, Obama said Bush and McCain were pushing the view that sitting down with adversaries is a sign of weakness.
"Understand that George Bush had a policy of not talking to North Korea and not talking to Iran and over the last eight years, they are stronger as a consequence of George Bush's foreign policy," Obama said.
McCain spokesman Tucker Bounds countered that Obama's approach showed poor judgment.
"Barack Obama's pledge to unconditionally bring Mahmoud Ahmadinejad to the world stage isn't 'new politics,' it's incredibly weak judgment," Bounds said in statement.
The campaign trail was roiled by news that Sen. Edward Kennedy, a leading Democrat and patriarch of an American political dynasty, was hospitalized after suffering a seizure at his Cape Cod vacation home. All three candidates expressed hopes for his speedy recovery.
The Massachusetts senator has endorsed Obama and has campaigned vigorously for him.
"We are going to be rooting for him, and I insist on being optimistic about how it's going to turn out," Obama told reporters during a visit to a hospital in Eugene, Oregon.