• CNN.com , Monday, April 28, 2008 10:30 AM

The battle for the Democratic presidential nomination between Sens. Barack Obama and Hillary Clinton is turning out to be the costliest ever, according to one ad-tracking service. The pair has spent a combined $110 million on TV ads up to now in a race that has gone for almost a year, says Evan Tracey of TNS Media Intelligence/Campaign Media Analysis Group.

"We're beyond what has been record spending in previous presidential nomination campaigns," he says, noting that John Kerry spent close to $20 million to get the nomination four years ago, while Obama has already shelled out almost $70 million. He outspent Clinton on ad buys in Pennsylvania by slightly more than 2-to-1, and Tracey says he is doing the same in Indiana and North Carolina, which hold primaries May 6.

Still, it hasn't necessarily worked in Texas, Ohio and then Pennsylvania. Tracey adds: "Clinton doesn't have as much money ... but if she's allowed to dictate the tempo, she can make up for that lack of spending."

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