President Joe Biden expressed doubts about Vice President Kamala Harris’ chances of defeating Donald Trump in the election. The 81-year-old president ultimately decided to drop his own re-election campaign and endorsed his running mate on Sunday, but three aides familiar with discussions about his plans told Axios that Biden and his senior advisers are worried that Harris isn’t up to the challenge of taking on the Republican nominee. “Harris’ time as vice president has been occasionally rocky, defined in part by large staff turnover, retreating from politically risky responsibilities, and mock…