Vice President Joe Biden and Rep. Paul Ryan (R-Wis.) are squaring off at 9 p.m. ET tonight in Danville, Ky. For a primer on what's in store, check out our viewer's guide, or go back in time and relive 7 great moments from past VP debates. In light of Obama's lackluster performance in the first presidential debate last week, tonight's VP face-off is considered a must-win for Biden, though some pundits say even a victory for Biden won't be enough to help Obama. And don't miss: 6 ways Joe Biden can trounce his GOP opponent, and 6 ways Paul Ryan can prevail.

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