In a poll to be released later today, CBS News finds:

While most voters (67 percent) say their vote will be based mostly on the candidate at the top of the ticket, 30 percent said that the choices of their vice presidential running mates will have a great deal of influence on their decision. That's twice the number who said the VP picks would matter in 2000, when George W. Bush and Al Gore were preparing their campaigns.

According to the poll, voters who are still undecided are more apt than those currently favoring Barack Obama or John McCain to say the candidates' choices for vice president will be important to their vote. Forty-eight percent of those voters say the choices will influence their vote while 47 percent say they won't. And Independents are more likely than Democrats or Republicans to say the choice of vice presidential nominee will matter.

What these numbers should mean is that the post-announcement bounce could tell us a lot about where this race is headed into the fall.