KEY POINTS:
RHODE ISLAND - US presidential candidate Barack Obama accused rival Democrat Hillary Clinton of using the politics of fear after her campaign released an ad suggesting that Obama lacked enough security experience to keep America safe.
"There are those who are telling you not to believe. There are those who are trying to feed your fear and your cynicism and your doubts," the first-term Illinois senator told supporters in Providence, Rhode Island.
Obama is seeking to raise doubts about Clinton's judgment, especially around her 2002 Senate vote to authorise the war in Iraq, in response to the television ad. It suggested Obama would not be able to handle a national security crisis.
The jabs over national security come as the candidates race toward Tuesday's primary votes in Ohio and Texas, big states that are considered must-wins for Clinton.
"We've been talking about change from the start of this campaign," Obama said. "Real change isn't voting for George Bush's war in Iraq and then telling the American people it was actually a vote for more diplomacy."
The former first lady, speaking in Texas, argued she was the only Democrat who can go toe-to-toe with Republican front-runner John McCain, a former prisoner of war who has been a strong advocate of larger troop numbers in Iraq.
"My opponent is now saying that raising national security in this election is fear-mongering ... . This is a wartime election and it matters who we put into the White House," the New York senator said.
"If Senator Obama is unwilling to engage me over national security, how is he going to engage Senator McCain?" Clinton told reporters later in the day.
National security has taken on a central role between the Democratic rivals as Obama, who has beaten Clinton in 11 consecutive contests, seeks to seal the nomination in Tuesday's nominating contests.
Clinton, once the unambiguous front-runner for the Democratic nomination, has seen her lead vanish in Ohio and Texas in recent weeks.
A Reuters/C-SPAN/Houston Chronicle poll released on Saturday showed Obama with a slim lead in Texas, 45 per cent to Clinton's 43 per cent, within the margin of error.
But she has maintained an edge among Hispanics in Texas, who could make up a third or more of voters there, and will look for support among women and older voters.
In Ohio, the candidates were dead even at 45 per cent.
The Clinton ad seized on Americans' security fears.
"It's 3am and your children are safe and asleep. But there's a phone in the White House and it's ringing. Something's happening in the world. Your vote will decide who answers that call," the ad's narrator says.
Obama's campaign responded with its own ad that questioned Clinton's vote in support of the Iraq invasion, arguing judgment was more important than experience.
"Senator Clinton is right when she says she's been tested on national security, but it's a test she has resoundingly failed," said Obama campaign spokesman Bill Burton.
In Texas, Clinton dug into Obama's record.
"He chairs a subcommittee on Nato, which is a major ally in the war in Afghanistan. But he failed to hold a single substantive hearing on Afghanistan or anything else," she said in prepared remarks. "He talks about these issues, but then he goes missing in action."
With the US economy faltering and resistance to international trade deals rising, the Iraq war has so far not played the same central role in the 2008 presidential race as it did in 2004. But it could well re-emerge on centre stage.
McCain, the Arizona senator who has a wide lead over rival Mike Huckabee, took the day off from campaigning.
McCain led the former Arkansas governor 58 per cent to 23 per cent in Ohio in Saturday's poll, and 54 per cent to 31 per cent in Texas.
- REUTERS
