The two presumptive favorites for 2008 presidential nominations each face enormous challenges, according to the new Cook Political Report/RT Strategies national poll. Indeed, their problems are almost mirror opposites of each other.For New York Sen. Hillary Rodham Clinton, Democrats and Democratic primary voters nationwide are highly receptive to her strengths and downplay her weaknesses. But among independents the view is considerably cooler, and is downright cold among Republicans, raising real questions of electability.On the other hand, if Arizona Sen. John McCain were as well regarded among Republicans and GOP primary voters as he is among independents and Democrats, he'd be...