Stage crew members prepare the set for tomorrow's vice presidential debate at Centre College October 10, 2012 in Danville, Ky. - 

This is, to judge by the punditry, the most consequential vice presidential debate ever. But when you look at the data on V.P. showdowns, they don't really move the needle.

Gallup's analysis of polls taken before and after the debates stretches back to 1976. There have been a number of memorable theatrical moments in the V.P. debates, like in 1988, when Lloyd Bentsen told Dan Quayle he was "no Jack Kennedy."

"You ought to care because it'll be fascinating to watch these two guys go at it together," said Frank Newport, editor-in-chief at Gallup. But, he added, "We just don't find much evidence looking at history that these vice presidential debates make much difference."

Unlike other election cycles where the dueling V.P. contenders enjoyed widely different popularity, Vice President Biden and Rep. Ryan are almost exactly matched, with 43 percent and 42 percent approval, respectively.