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In Ohio, the race remains a dead heat with Mr. McCain edging ahead 44 percent to 43 percent, revealing a slight decline for Mr. Obama. Whites back Mr. McCain by 11 percentage points, but his opponent has a 14-point lead among women.

In Pennsylvania, a state that has voted reliably Democratic in recent presidential elections, Mr. Obama’s lead has shrunk slightly since June. Mr. McCain now trails him by seven points, 49 percent to 42 percent. Mr. Obama lost the Pennsylvania primary to Sen. Hillary Rodham Clinton, and about a third to half of her supporters have told pollsters they either will vote for Mr. McCain or will not vote at all.

Democratic pollsters say their surveys are showing similar weaknesses in Mr. Obama’s numbers, blaming the shift on Mr. McCain’s fierce offensive campaign and the Obama campaign’s tepid ad responses.

“The central challenge for Obama is to start defining the choice in this race on his terms, and putting McCain on defense,” Democratic pollster Celinda Lake said last week in her analysis for the bipartisan Battleground Poll.