Republican John McCain and Democrat Barack Obama faced off on Wednesday in a possible prelude to a US presidential election battle, tangling over whether Iraq would be prey for al Qaeda if US troops are withdrawn.

Democrat Hillary Clinton, who needs big wins in Texas and Ohio next Tuesday to salvage her struggling candidacy, declared herself optimistic about her chances following her final debate with Obama on Tuesday night in Cleveland.

"What keeps me optimistic is the success I've had thus far and what I think the prospects are for Tuesday. People have just been really rallying to my candidacy," she said.