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Gallup has some numbers that matched up Democrats Hillary Clinton & John Kerry against Republicans Rudy Giulliani & John McCain.
PERSONAL FAVORABILITY RATING
Except for John Kerry, whose numbers have went to a net negative 42/48, all of the candidates have a positive favorability rating.

HYPOTHETICAL 2008 MATCHUPS
Both Democrats lose to the Republicans but Hillary does much better than Kerry, who loses by double digits. Hillary gets 80% of Democrats, 40% of Independents, and 11-12% of Republicans. John Kerry only gets 3/4 of Democrats, 35-38% of Independent, and 7-10% of Republicans.

NOTE: (July 25-28, 2005. 922 registered voters, with a margin of error ±4%.)