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He and Mr. Obama repeatedly addressed themselves through the television cameras to “Joe.”

The candidates also clashed on spending and energy independence. At one point, when talking about energy, Mr. McCain said he wanted to decrease reliance on oil from the Middle East and Venezuela, but said Canadian oil is fine.

But his own support for a low-carbon fuel standard would actually discourage use of Canadian oil, which is considered “dirty” because much of it relies on oil shale and producing a barrel of oil from shale releases more greenhouse gases than convention oil from the Middle East.

Mr. Bush’s popularity has slid in the past four years, and Mr. Obama has repeatedly linked Mr. Bush to Mr. McCain. In the first two debates, he pointed to administration policies and said the two Republicans share the same economic philosophy that has led to the Wall Street meltdown.

The earlier debates covered little on a key bit of unfinished business from Mr. Bush’s two terms — Social Security. And no mention about immigration was made in any of the debates.