Biden’s position tenuous amid reports campaign secretly testing Harris’s popularity

Reports come as Biden mistakenly refers to Ukraine president Volodymyr Zelenskiy as President Putin' at Nato summit

Joe Biden's position appeared shaky on Thursday, amid reports that his aides and advisers were discussing how to persuade him to leave the presidential race while his own campaign was secretly testing Kamala Harris's popularity, suggesting it was preparing for that very scenario.

Then just before the US president was scheduled to face journalists at a potentially pivotal news conference marking the end of Nato's 75th anniversary summit, he spoke to reporters there and mistakenly referred to Ukraine's president, Volodymyr Zelenskiy, as President Putin".