John Kerry knows from negative GOP campaigning, and last night, Rachel Maddow invited him to discuss John McCain's attacks on Barack Obama's character. Kerry is disgusted by the McCain camp's obsession with Bill Ayers, and brought up all the questionable folks McCain has palled around with. From the lobbyist who worked for Saddam Hussein to Scooter Libby, McCain has crossed paths with some bad folks, Kerry points out, and that doesn't make him any more guilty than Obama is for knowing Ayers. The negative campaigning is "insulting to Americans," he adds. And Americans seem to agree: a NY Times poll shows that voters are three times more likely to have changed their view of McCain from positive to negative. Clip above.No Turning Back? [MSNBC] Poll Says McCain Hurts His Bid by Using Attacks [NY Times]