KINGSTREE, South Carolina (Reuters) - Democrat Barack Obama on Thursday defended the fierce tone of his recent exchanges with U.S. presidential rival Hillary Clinton and said he was forced to fight back because of her campaign’s disregard for the truth.

Obama, an Illinois senator, said he was battling a “tough, well-honed political machine” operated by Clinton and her husband, former President Bill Clinton, but did not think their escalating feud would hurt the party in November’s election.

“One principle that I think we want to firmly establish is, if people are making false assertions about my record, we will answer them,” Obama, who would be the first black U.S. president, told reporters.

The top two contenders for the Democratic nomination have engaged in a widening war of words, including a debate on Monday in which they traded a series of harsh and sometimes personal attacks.

Obama ran a tough radio ad accusing Clinton, a New York senator who would be the first woman U.S. president, of being willing to “say anything to get elected.” The campaign used a fund-raising e-mail by Obama’s wife, Michelle, to level similar charges.

Obama said the radio spot was in response to Clinton’s ad, which he said distorted his comments about Republican ideas.

The Clinton radio ad used Obama’s quote in Nevada last week that Republicans had been “the party of ideas” in recent years and implied he supported those ideas. Obama says he never claimed to like them -- a view backed by several independent analysts.

WRONG ASSERTIONS

“When you run an ad making assertions that everybody who has looked at it says are wrong, you know they say it is wrong, and you still make it, then that would indicate that you are not that concerned about accuracy or the truth,” Obama said.

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The Clinton campaign pulled its radio ad on Thursday in what a spokesman said was a planned rotation, replacing it with an ad featuring Bill Clinton praising his wife’s record and experience.

An Obama spokesman said his campaign was telling stations to withdraw their radio ad in response.

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Obama, who entered the campaign promising a more uplifting message of hope about U.S. politics, said he expected he and Clinton would put aside their differences once someone claimed the nomination.

“I am confident the entire Democratic Party will rally around the eventual nominee,” he said. “I don’t feel the candidates are being bloodied up. This is good practice for me.”

Polls show Obama with a comfortable lead on Clinton in the latest Democratic battleground of South Carolina, which votes on Saturday in a primary where more than half of the likely voters are expected to be black.

“The Clinton operation is a tough, well-honed political machine, built up over the course of 20 years. We have always been the underdogs in this campaign,” he said.

Obama has complained specifically about comments from Bill Clinton, including his charge that Obama’s consistent opposition to the Iraq war was “a fairy tale.”

“The only thing I am concerned about is when he makes misstatements about my record. That’s what I’m seeking to correct,” Obama said.

(Editing by Chris Wilson and Todd Eastham)