'I'm not suggesting there's never a time to talk, but I'm suggesting now is not the time ... to talk with Raul Castro,' Mr Bush told a White House news conference on Thursday after being asked about Mr Obama's willingness to meet with the new Cuban president, Mr Raul Castro.

'It will send the wrong message. ... It will give great status to those who have suppressed human rights and human dignity,' Mr Bush added, saying there was no difference between Mr Raul Castro and his brother, Fidel, who recently stepped aside as president because of ill health.

It was the first major instance of Mr Bush injecting himself into the presidential race to choose who will succeed him in the November election, with his unpopular Iraq war a major debating point on the campaign trail.

Asked about Mr Obama's remark during a recent debate with rival Hillary Clinton that he advocated withdrawing from Iraq but might send troops back to avoid Al-Qaeda forming a base there, Mr Bush said US forces had been fighting in Iraq for four years to prevent that from happening.

'I believe Senator Obama better stay focused on his campaign with Senator Clinton, neither of whom has secured their party's ... nomination yet,' Mr Bush added.

Mr Obama quickly fired back, saying: 'The American people aren't looking for more of a do-nothing Cuba policy that has failed to secure the release of dissidents, failed to bring democracy to the island and failed to advance freedom for 50 years.'

Mr Obama repeatedly has advocated meeting without preconditions with leaders of such hostile nations as Cuba and Iran, saying current US policy is not working and it is time for a fresh look at ways to improve relations.

'The next president has a job to do to repair our image and to send a signal ... that a new era is being ushered in and that we are not afraid to talk to anybody, including those who we have grave problems with,' Mr Obama said.

Mccain, Obama trade barbs
Mr Bush's comments on Mr Obama's policies reflected how closely he is watching the campaign and how eager he is to guard his legacy. He has yet to formally endorse the Republican front-runner, Arizona Senator John McCain, but is expected to do so.

Mr McCain, a strong Iraq war supporter and former prisoner of war in Vietnam, continued his attacks on Mr Obama on Thursday, criticising the Illinois senator for pledging to withdraw from Iraq.

'The decision to unilaterally withdraw from Iraq and set a date for withdrawal would lead to chaos,' Mr McCain told reporters at a Houston airport. 'Al-Qaeda will announce to the world that they have defeated the United States of America.' Mr Obama fended off the criticism.

'The American people aren't looking for tough talk about fighting for 100 years in Iraq, because they know we need to end this war, finish the job in Afghanistan, and take the fight to Al-Qaeda,' he said.

Mr Obama's rejection of what he calls conventional thinking in Washington has put him in the driver's seat to seize the Democratic nomination from Mrs Clinton, long considered the inevitable nominee.

The exchange came as a new Pew Research Center poll showed Mr Obama with a broad-based advantage over Mrs Clinton before critical March 4 contests in Texas and Ohio to select a Democratic presidential candidate for the November election.

The poll found Mr Obama leading Mrs Clinton 49 per cent to 40 per cent among Democratic primary voters. But 70 per cent of Democratic voters now see Mr Obama as the likely nominee, including more than half of Mrs Clinton supporters, the poll found.

Mrs Clinton, who has strong support among low-income voters, toured the economically struggling Appalachian foothills of southern Ohio on Thursday to tout her proposals to help lower-income and middle-income families.

The former first lady and New York senator announced she had raised US$35 million (S$49 million) in the past month, but Mr Obama reportedly was poised to surpass her in fundraising again.

Mr Obama has consistently led Mrs Clinton in fundraising, an advantage that showed in advertising figures for Texas and Ohio. Media research firm Nielsen reported on Thursday that Mr Obama aired 80 per cent more ads in Ohio than Mrs Clinton between Feb 1 and Feb 24, and nearly 50 per cent more in Texas. -- REUTERS