Washington - Former US secretary of state Colin Powell, the nationally respected military leader who served in the first Republican administration of President George W Bush, turned the tables on his own party Sunday by endorsing Democratic presidential candidate Barack Obama.

The announcement by Powell on NBC's weekly Meet the Press came as a further blow to Republican candidate John McCain, 72, the Vietnam war hero who continues to lag behind Obama in opinion polls with the November 4 elections looming.

It also served to shore up Obama's lack of military and national security experience - major shortcomings in the eyes of many voters. McCain has used his own experience as a navy pilot and prisoner of war in Vietnam to undermine Obama on these issues.

In his endorsement, Powell cited Senator Obama's "ability to inspire" and the "inclusive nature of his campaign." He described the 47-year-old son of a white mother from Kansas and a black father from Kenya as a person with "both style and substance. I think he is a transformational figure."

Saying the US needed a "generational change," Powell added, "I firmly believe that at this point in America's history, we need a president ... who will not just continue basically the policies that we have been following in recent years."

McCain dismissed the importance of the move, saying he had the backing of four other former secretaries of state and more than 200 retired army generals and admirals.

"This doesn't come as a surprise," McCain said in broadcast remarks.

Morris Reid, a Democratic strategist, told NBC that Powell's endorsement "underscored" Obama's leadership ability, but warned that "this is still a 2 to 5 per cent race and we have to get people to the polls."

At a rally in Fayetteville, North Carolina on Sunday, Obama said he was "deeply humbled" by Powell's support and described him as "a great soldier, a great statesman and a great American."

Obama told the cheering crowds, "He knows, as we do, that this is a moment where we all need to come together as one nation ­ young and old, rich and poor, black and white, Republican and Democrat."

Powell's move carries the weight of more than 40 years of service to his country, much of it as a high-profile moderate Republican.

It was expected to make a difference with centrist Republicans and especially Republican women in the nation's suburbs worried that McCain would re-criminalize abortion, analysts said.

Powell was the first African American to hold the top US diplomatic post as secretary of state. He served as a US Army major in Vietnam, chairman of the joint chiefs of staff during the first Gulf War, national security advisor to the late president Ronald Reagan and later commander of the US Army's Forces Command.

He was a reluctant supporter of Bush's 2003 invasion of Iraq, and his presentation before the United Nations just months before the 2003 invasion of what later turned out to be hyped intelligence material, went a long way to convincing skeptics about the danger posed by Saddam Hussein.

Powell also advocated for use of overwhelming force to overcome Iraqi resistance, achieve the mission and withdraw quickly, but was overruled by then secretary of defence Donald Rumsfeld.

Powell, once seen as a major presidential possibility, said his endorsement was a tough decision. He had informed both senators that he would decide neither on the basis of a long friendship with McCain nor on the basis of shared ethnicity with Obama.

What tipped the scales to Obama for Powell was the "final exam" - weeks of the global economic crisis, when McCain had a "different approach to the problem ... almost every day" and gave the impression he lacked "complete grasp" of the issue, Powell said.

Obama, on the other hand, has shown "intellectual curiosity, depth of knowledge" and calm leadership.

Powell was also unsettled by McCain's choice of someone not ready to be president as his running mate, Alaska Governor Sarah Palin.

Powell said he worried that McCain's campaign had become "narrower and narrower" and moved "even further to the right." He faulted McCain for demagoguery on "issues that are not really central to the problems" such as targeting Obama with robo-calls trying to link the Democratic senator with terrorism.

"I would have problems with two more conservative appointments to the Supreme Court," Powell said, referring to the likelihood that the next president will appoint at least two court replacements during the coming years.

Powell was worried about how some Republicans have tried to discredit Obama with the false claim that he is a Muslim.

"(Obama's) not a Muslim. He's a Christian. But the right answer is, 'What if he is?' Is there something wrong with some seven-year- old Muslim American kid believing he or she can be president?"

Powell said that beyond strong substance, Obama had "met the standard of being an exceptional president" at a time when the country desperately needed a leader who can convey "a new image of American leadership" and restore a "sense of confidence" in America both at home and abroad. (dpa)