In his first month out campaigning, Joe Biden has been the clear front runner in the polls, securing endorsements from many Democratic office holders, and solidifying support with key constituencies across the party. CNN Political Director David Chalian reflects on one gnawing question that continues to surround his candidacy. Is Biden's frontrunner status rock solid or is there a soft underbelly to it that makes it truly vulnerable to a credible challenger?To learn more about how CNN protects listener privacy, visit cnn.com/privacy

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