Embattled and increasingly bitter, Joe Biden angrily lashed out on Monday when he was asked – yet again – if he'd considering stepping down as the presumptive Democratic nominee for president. 'Who else do you think could step in here and do this?' he spat during a phone-in interview with MSNBC hosts Joe Scarborough and Mika Brzezinski. 'Run against me!' he said, daring Democrat rivals. 'Challenge me at the convention.'