Joe Biden makes his first public comments since he struggled in the debate on Thursday
Directly addressing concerns about his age and saying "when you get knocked down, you get back up"
Democrats have expressed unease at the president's shaky debate performance
With party insiders saying some of his answers triggered panic
Biden's campaign insists the president will not step down as the Democratic candidate
That he will debate Donald Trump again in September
At a campaign rally in Virginia, Trump told the crowd the debate was a "big victory" for him
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Said he does not think Biden will bow out of the race