The Trump presidency has been dogged with allegations of collusion with Russia since before it formally began. Details are, however, now emerging about President Trump's relationship with Ukraine's new president, Volodymyr Zelensky. Trump reportedly phoned Zelensky on July 25, and while discussing a range of topics, asked him to reopen a probe into Democratic presidential candidate Joe Biden. Ukraine had been investigating a decision Biden had made while vice president to pressure the country into dismissing its top prosecutor, Viktor Shokin, who was investigating the Ukrainian natural gas company Burisma Holdings. Biden's son, Hunter, was on the company's board at the time. If Trump did ask Zelensky to reopen the probe, it would constitute a request by a sitting president to a foreign leader to investigate one of his political opponents. With us to discuss is Michael Isikoff, chief investigative correspondent at Yahoo News. He’s the co-author of Russian Roulette: The Inside Story of Putin's War on America and the Election of Donald Trump.