"I'm not sure that (Joe Biden) can quiet this drumbeat about his viability, about his age and capacity at this point. No matter what he does," a US-based political analyst says after a string of verbal gaffes at a major summit threw a harsh new spotlight on the US president's fitness. "Every flub is magnified tenfold," Matt Dallek, professor of Political Management at George Washington University, adds. "It's become a structural problem, I think, for his campaign." SOUNDBITES