Democrat Barack Obama praised his rival John McCain for trying to tone down the vitriol of the US presidential race but pressed ahead yesterday with an effort to cast the Republican as out of touch on the economy.

McCain faced fresh troubles after an ethics inquiry found that his running mate, Alaska Governor Sarah Palin, abused her authority in a matter involving the firing of a state trooper.

With just over three weeks left before the November 4 election, polls showed a growing lead for Obama, 47, as voters anxious about turmoil on Wall Street have given the Democratic candidate higher marks for economic leadership.

A Newsweek poll published on Friday showed Obama, an Illinois senator, ahead of the 72-year-old Arizona senator by 52 per cent to 41 per cent.

A month ago, that poll had the two candidates tied at 46 per cent. Other polls in the most contested states have also shown a swing toward Obama.

The Palin ethics scandal cast a cloud over McCain's controversial choice of a running mate and threatened to overshadow Republican efforts to raise questions about Obama's character.

With supporters on both sides hurling sometimes nasty comments, McCain urged one of his backers to take a milder tone and in doing so, appeared himself to defend Obama's character.

When one woman at a town-hall style meeting on Friday said she did not trust Obama and raised a false assertion that he was an "Arab," McCain replied that Obama was a "decent person and a person that you do not have to be scared (of) as president of the United States."

But McCain said he thought he would be a better president than Obama.

Touring neighbourhoods in Philadelphia, Obama thanked McCain for having tried "tone down the rhetoric of his town-hall meeting."

But in an indication of the anger running high on both sides, the crowds at Obama's Philadelphia rallies booed loudly at the mere mention of McCain's name. "I'm one who believes that we can all respect each other, even when we disagree, especially when it comes to a veteran of our wars because those folks lay down their lives to protect us," Obama said, referring to McCain's service in Vietnam, where the Republican endured torture as a prisoner of war.

Pushing a line of attack that seems to have helped Obama build an advantage, Obama said McCain "doesn't really seem to get what's going on" with the financial crisis that has prompted deepening anxiety about the economy.

Obama mocked a McCain adviser for telling reporters amid a week of panic-selling on Wall Street that he didn't think it made sense for the campaign to speak daily on the markets.

The campaign should not become a "CNBC news show on the stock market," said McCain campaign manager Rick Davis, according to Politico magazine.

"I can't imagine a situation where on a daily basis the campaign would put out a statement about what the market was doing," Davis was quoted as saying during a conference call with journalists.

"Yesterday, Senator McCain's campaign manager actually said that Senator McCain wasn't talking about the market because there's just not much a candidate for president can say - and they aren't sure what he'd say each day even if he did talk about it," Obama said.