St Paul, Minnesota/Washington - US President George W Bush called Republican presidential hopeful John McCain the candidate who "understands the lessons" of the terrorist attacks of September 11, 2001, in an address by satellite to the Republican Party's nominating convention on Tuesday.

Bush, whose low approval ratings have kept him from playing a larger role in the McCain campaign to date, was originally to appear in person Monday at the St Paul, Minnesota convention. He cancelled the visit due to Hurricane Gustav's arrival on the Gulf Coast.

Bush characterized McCain as an independent thinker who is best able to lead the country in a time of war, and touted McCain's military history as a Vietnam War fighter pilot who was captured and tortured for five years in a Hanoi prison.

"We live in a dangerous world. And we need a president who understands the lessons of September 11, 2001: that to protect America, we must stay on the offense, stop attacks before they happen, and not wait to be hit again," Bush said.

"I have stood in the ruins of buildings knocked down by killers ... I know the hard choices that fall solely to a president," Bush said, referring to the collapse of New York's World Trade Center towers in the 2001 attacks. "John McCain's life has prepared him to make those choices. He is ready to lead this nation."

Republicans have hammered Democratic presidential nominee Barack Obama, who was elected to the Senate in 2004, for being unprepared to lead the country. Bush made no specific references to Obama in his speech.

With Bush's approval ratings hovering below 30 per cent, the McCain campaign has been keeping the president at arms-length in the race. McCain, 72, has appeared publicly with Bush only once - at a private fundraiser - since the president endorsed him in March.

Bush headlined a convention night that included independent Senator Joe Lieberman, a one-time Democratic vice presidential candidate, and actor and former presidential candidate Fred Thompson.

Democrats used their own convention last week in Denver, Colorado, to link McCain's policies to those of the Bush presidency, arguing that McCain simply brings "more of the same."

"The man George Bush needs may be John McCain, but the change America needs is Barack Obama," Obama campaign manager David Plouffe said in a statement Tuesday.

The McCain campaign has touted their candidate's long-held reputation as a maverick unafraid to go against his own party, and Bush alluded to that track record in his speech.

"John is an independent man who thinks for himself. He's not afraid to tell you when he disagrees - believe me, I know," quipped Bush, who has tussled with McCain on issues including torture and global warming.

"No matter what the issue, this man is honest and speaks straight from the heart," Bush said.

While Bush was unable to attend, his father and former president George HW Bush received a long standing ovation as he entered the convention hall with his wife, Barbara.

The former president did not address the convention, but the party did run a video tribute of Bush, as well as of former Republican presidents and icons Abraham Lincoln, Teddy Roosevelt and Ronald Reagan.

McCain will accept the Republican Party's nomination on Thursday.