Republican vice-presidential nominee Sarah Palin has accused Democratic candidate Barack Obama of associating with terrorists, marking a shift to a nastier tone in the White House race.
The remark was dismissed by Mr Obama as "gutter politics" but appeared to reflect an effort by senator John McCain's campaign to target his opponent's judgment.
It came shortly after the Mr McCain campaign called Mr Obama a liar.
Ms Palin said "there is a time when it's necessary to take the gloves off and that time is right now".
She cited a New York Times report that Mr Obama had had links with a former member of a 1960s militant group.
The Alaska Governor attacked Mr Obama over his links to Bill Ayers, a founder of the militant group Weather Underground, that waged a violent campaign against the Vietnam War.
The group was blamed for a number of bombings in the US in the 1960s.
Ms Palin described Mr Obama as someone who saw the US "as being so imperfect ... he is pal-ing around with terrorists who would target their own country".
Mr Obama, who served on a charity board several years ago with Mr Ayers - now a professor at the University of Illinois - has denounced his radical activities.
Commentators say Ms Palin's attack forms part of a broader Republican strategy to attack Mr Obama's character.