Tougher TV ads attacking U.S. Democratic presumptive nominee Barack Obama appear to have paid dividends for Republican John McCain.

A new poll had the two leading candidates for the U.S. presidency tied on the weekend.

Obama had held a nine-point lead in the Gallup Poll tracking survey as of July 26, but that lead completely eroded by Saturday, when the two were tied at 44 per cent.

McCain had vowed to avoid the types of negative tactics that George W. Bush successfully used against him in the 2000 Republican primary.

However, he has picked up some new advisers who had worked on Bush's re-election campaign in 2004 -- a campaign seen as one of the most negative in recent memory.

The new ads targeted Obama's trip to Iraq and Afghanistan last month.

McCain, a four-term senator from Arizona and a Vietnam War hero, presented himself during the Republican primaries as a national security candidate.

He charged that Obama's promise to withdraw U.S. forces from Iraq within 16 months of taking office was tantamount to choosing to lose the war in order to win the presidency.

McCain's team then ran an ad that accused Obama of not visiting wounded U.S. troops in Germany because he couldn't take television cameras along -- a claim that appears to be false.

Another commercial blended images of Obama with pop figures Britney Spears and Paris Hilton, trying to paint the first-term senator from Illinois as a celebrity without the experience necessary to be commander-in-chief.

Kathy Hilton, Paris's mother, blasted the ad. She called it "a complete waste of the money John McCain's contributors have donated to his campaign." Hilton and her husband had donated $4,600 to McCain's campaign earlier this year.

Obama stood accused of engaging in racial politics by claiming McCain and other Republicans would try to frighten Americans because he didn't look like past U.S. presidents. Obama, if elected, would be the first black U.S. president.

An Internet ad calls Obama "The One" in a voiceover and features clips of Obama seemingly describing himself and his presidential fight in overblown language. The final image is Charlton Heston as Moses parting the Red Sea in the movie "The Ten Commandments."

Obama called McCain's tactics cynical but not racist.

"In no way do I think John McCain's campaign was racist. I think they are cynical," Obama said Saturday. "Their team is good at creating distractions and engaging in negative attacks."

With files from The Associated Press