Allies and rivals have reacted to the choice of Sarah Palin as Republican vice-presidential nominee, calling her a "new voice" in the election campaign.
President George W Bush said Mrs Palin was a "wonderful example", while Democratic nominee Barack Obama said she would add interest to the campaign.
But he added that the Republican policy message remained "more of the same".
Mother-of-five Mrs Palin, 44, was unveiled on Friday, a surprise choice as Mr McCain's running mate.
As governor of Alaska, she has been credited with reforms during her first term in office, but remains relatively unknown in US politics.
Mr Obama and his running mate, Joe Biden, were back on the campaign trail on Friday after their candidacies were confirmed at the Democratic party convention in Denver, Colorado.
On board a flight to Pennsylvania, the pair said Mrs Palin's selection was "yet another encouraging sign that old barriers are falling" in politics.
"While we obviously have differences over how best to lead this country forward, Governor Palin is an admirable person and will add a compelling new voice to this campaign," they said in a statement.
Mr Obama told reporters that Mrs Palin had a "terrific" personal story and would help make a case for the Republicans, but added: "Unfortunately, the case is more of the same."
A day earlier, John McCain praised Mr Obama's achievement in becoming the first African American selected by a major party as presidential nominee, taking out a TV advert to congratulate his rival during coverage of the Democratic convention.
'Proven reformer'
The warm words from Mr Obama and Mr Biden were in contrast to earlier remarks by Obama campaign spokesman Bill Burton, who suggested Mr McCain's choice of Mrs Palin was irresponsible.
SARAH PALIN
Mr Obama said Mr Burton's remarks had been a "hair trigger" reaction to the announcement and did not reflect his or Mr Biden's true sentiments.
Meanwhile, President Bush has hailed Mrs Palin as "a proven reformer" and a "wise steward of taxpayer dollars and champion for accountability in government".
"Governor Palin's success is due to her dedication to principle and her roll-up-your-sleeves work ethic and serves as a wonderful example of the spirit of America," he said.
Mr Bush said that that by selecting "a working mother with a track record of getting things done", Mr McCain had shown his commitment to reforming Washington.
Mrs Palin is the second female US vice-presidential candidate for a major party. The first was Democrat Geraldine Ferraro, who ran alongside Walter Mondale in 1984.
Analysts say Mrs Palin may have been chosen by the McCain campaign as a tactic to win over those Democratic women voters who were disappointed by the defeat of Mrs Clinton by Mr Obama.
Audacious choice
Some commentators are calling Mr McCain's running mate decision the strangest since Dan Quayle, George Bush Sr's young and untested pick in 1988.
From Wasilla to the White House?
The fact that she is a woman and a fresh face on the national scene is obviously a potential plus, bringing an excitement and newsworthiness to the Republican ticket, the BBC's Justin Webb reports.
However, Mrs Palin's less than two years in office would undercut one of the McCain campaign's central criticisms of his Democratic rival Barack Obama: that he is too inexperienced to be commander-in-chief.
The choice seems an audacious, perhaps slightly desperate, effort to bring youth and vigour to the Republican ticket, our North America editor adds.
Mrs Palin is seen as a strong conservative, fiscally and socially.
She also opposes abortion and her baby son has Down's syndrome.
The influential conservative lobby Family Research Council said Mrs Palin was an "outstanding choice" as vice-presidential candidate.
Abortion has been a key issue cited by Republican voters who feel Mr McCain is not conservative enough.
Mrs Palin's electoral chances may be harmed by the fact that she was placed under investigation in Alaska by state lawmakers at the end of July.
She sacked a public safety commissioner and the allegation is that she removed him because he had not fired a state trooper who is Mrs Palin's former brother-in-law, and who is in a custody battle with her sister.
She says she has "nothing to hide" and is "cool" about the investigation.
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